feat(tmproto): typed per-provider credentials for ServiceAccountAccess - #477

Summary

Closes #51.

AssetAccess with Method == service_account was the one payload left in the SDK where credentials were carried as opaque map[string]any — the bearer-equivalent credential blob where typing matters most. This PR types it for the two providers the schema recognizes (gcp, aws) while keeping forward compatibility for any other provider.

What changed

New typed credential structs (tmproto/artifact.go), matching the issue's proposed shape:

type GCPServiceAccountCredentials struct {
    ClientEmail string `json:"client_email"`
    PrivateKey  string `json:"private_key"`
    ProjectID   string `json:"project_id,omitempty"`
    TokenURI    string `json:"token_uri,omitempty"`
}

type AWSServiceAccountCredentials struct {
    AccessKeyID     string `json:"access_key_id"`
    SecretAccessKey string `json:"secret_access_key"`
    SessionToken    string `json:"session_token,omitempty"`
    Region          string `json:"region,omitempty"`
}

No published JSON Schema exists for these shapes in adcp/schemas/ (this is TMP-specific asset-access credential material, not core AdCP), so per docs/sdk-typing-policy.md the issue's proposed shape is authoritative here — I checked the schemas directory and the public spec site before proceeding.

Provider-aware constructors:

func NewGCPServiceAccountAccess(creds GCPServiceAccountCredentials) AssetAccess
func NewAWSServiceAccountAccess(creds AWSServiceAccountCredentials) AssetAccess

AssetAccess.Credentials becomes typed via a discriminated interface, not just for these two providers but with an explicit escape hatch for others:

type ServiceAccountCredentials interface {
    ProviderTag() string // "gcp", "aws", ...
}

GCPServiceAccountCredentials and AWSServiceAccountCredentials implement it. A third type, RawServiceAccountCredentials{Provider string; Fields map[string]any}, is the forward-compat fallback for any provider this SDK doesn't type yet — it round-trips the wire object losslessly instead of failing to decode. AssetAccess.UnmarshalJSON dispatches on the wire "provider" value (decodeServiceAccountCredentials) to pick the concrete type. This mirrors the dispatch-with-fallback pattern Assets.UnmarshalJSON already uses for the asset "type" discriminator (typed struct for known values, UnknownAsset passthrough otherwise) — same shape, applied to the second discriminated field in this file.

NewServiceAccountAccess(provider string, credentials map[string]any) (the pre-existing constructor) is kept as the explicit "I don't have a typed struct for this provider" path — it now wraps its argument in RawServiceAccountCredentials. Prefer the typed constructors for gcp/aws.

Redaction, mirroring AssetAccess's existing String()/GoString() pattern exactly (both delegate to a private redacted() method building Type{Field:val,...,<redacted>}):

• GCPServiceAccountCredentials: redacts PrivateKey; keeps ClientEmail, ProjectID, TokenURI visible.
• AWSServiceAccountCredentials: redacts both SecretAccessKey and SessionToken (a session token is bearer-equivalent — sufficient alone to act as the principal); keeps AccessKeyID, Region visible.
• RawServiceAccountCredentials: redacts the entire field map, since the SDK doesn't know an untyped provider's shape well enough to tell secret fields from non-secret ones.

I did make one deliberate departure from "same pattern": AssetAccess.redacted() itself is a blanket redaction (only Method survives) because at that layer the SDK can't yet distinguish secret vs. non-secret sub-fields generically. For the new typed structs we do have that information per-field, so — per the issue's explicit ask to keep non-secret identifiers "visible for debuggability" — the credential structs redact only the actual secret fields rather than blanket-redacting like AssetAccess does. The mechanism (String/GoString → private redacted() → Type{...,<redacted>} format) is identical; the granularity is finer because more information is available.

Tests (tmproto/artifact_test.go, tmproto/robustness_test.go)

• Round-trip tests for GCP and AWS with realistic (fabricated) credential shapes, including AWS SessionToken (STS temporary credentials are a common real-world shape) — construct via constructor → json.Marshal → exact wire-format assertion (assert.JSONEq) → json.Unmarshal → type-assert the decoded Credentials back to the concrete typed struct → assert.Equal against the original.
• TestAssetAccess_ServiceAccount_UnknownProvider_RawFallback: an unrecognized provider ("azure") still round-trips via RawServiceAccountCredentials instead of erroring.
• Redaction tests for both new structs and for RawServiceAccountCredentials assert the actual secret substring is absent from %s/%v/%+v/%#v output — not just that "some redaction happened" — while separately asserting the non-secret identifiers are present, proving the field-level split actually works both ways.
• Updated the two existing tests that literally constructed AssetAccess{..., Credentials: map[string]any{...}} to use the new typed constructors, since that field's type changed.

Verification

cd tmproto && go build ./... && go vet ./... && golangci-lint run ./... && go test -race -count=1 ./...

All pass, 0 lint issues. Also go build ./... / go vet ./... at repo root (root module doesn't reference AssetAccess, so it's unaffected). Searched the whole repo for other call sites constructing/reading AssetAccess with Method == service_account — none outside tmproto itself.

Honesty notes (acceptance criteria vs. reality)

• Prerequisite check: confirmed AssetAccess already has the sum-type shape (Method/Provider discriminators, existing redacting String()/GoString(), custom MarshalJSON/UnmarshalJSON) this issue assumes — the disclosure-ladder PR (feat: type the full TMP content disclosure ladder #59) it was blocked on is merged.
• Schema check: there is no published JSON Schema for these credential shapes in this repo or (as far as I could find) on the public spec site, so I used the issue's proposed field shape as-is, per docs/sdk-typing-policy.md's "no schema → the issue's sketch is authoritative" fallback. Worth a maintainer sanity-check if a schema exists upstream that I didn't find.
• Breaking change: AssetAccess.Credentials changes type from map[string]any to the ServiceAccountCredentials interface, and NewServiceAccountAccess's behavior changes (now wraps in RawServiceAccountCredentials). This is a genuine breaking API change to public Go types, marked with ! in the commit/PR title per this repo's bump-minor-pre-major: true release-please convention (matches the precedent of PR "feat(tmproto)!: spec-correct HashURL + url_hash artifact-ref support"). No other package in this repo touches AssetAccess, so nothing else needed updating, but any external consumer pinned to tmproto pre-this-change would need to migrate.

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