feat(signing): ship signing/signingtest subpackage + ObserveOnly mode

[sujanchalla0510]

Closes #53.

Summary

• adcp/v3/signing/signingtest — NewTestAgent(t) builds a matched Ed25519 *signing.Signer + signing.MiddlewareOptions (a fresh StaticJWKSResolver seeded with the signer's public key, a fresh NewMemoryReplayStore(0), no revocation). SignAndSend(t, signer, handler, req) signs req (with CoverContentDigest: true) and delivers it to handler in-process via httptest.NewRecorder, returning the *http.Response. Together these collapse the ~30-line keypair/JWK/resolver/replay-store pattern middleware_test.go currently hand-rolls into two lines of setup + one line to send.
• MiddlewareOptions.ObserveOnly bool — a real behavioral branch in Middleware(), not a passthrough field. When true, VerifyRequestSignature still runs; on failure the request is logged at slog.LevelInfo (vs the normal slog.LevelWarn) with an observe_only=true attribute and passed to next.ServeHTTP with no VerifiedSigner in its context — i.e., treated exactly as unsigned. On success, behavior is unchanged.

ObserveOnly ↔ spec mapping

This maps to the AdCP transport spec's warn_for rollout stop (supported_for → warn_for → required_for, see Transport capability advertisement). Per spec: "A missing signature or a well-formed signature that fails verification or body binding MUST NOT establish verified-signer identity ... A partial or malformed Signature / Signature-Input pair always hard-rejects."

ObserveOnly implements that split precisely, not just "log and let everything through":

• A well-formed signature that fails verification (bad crypto, unknown/revoked key, expired window, replay, wrong content-digest, an unsigned request to a RequiredFor op, ...) is observed: INFO log, request passes through unauthenticated.
• A partial or malformed Signature/Signature-Input header pair — one header present without the other, or either header present but unparseable (*Error{Code: CodeHeaderMalformed}) — still hard-rejects with 401 even under ObserveOnly, because the spec says it "cannot be safely interpreted as either signed or unsigned traffic."

adcp/v3/signing/MIGRATION.md's "Step B — warn_for" section previously described this exact mode as not yet landed, with an OnReject-based logs-and-passes-through shim as the interim workaround (and a "delete this before enabling RequiredFor" warning). This PR replaces that shim guidance with the real ObserveOnly API throughout the migration guide (bootstrap → step B → common pitfalls → pre-enforcement checklist).

Module note

This targets adcp/v3/signing, not adcp/signing. Per the repo's README.md "Modules & versioning" table and MIGRATING.md, adcp (the pre-v3 module) is frozen at v2.1.1 and receives security backports only — adcp/v3 is the actively developed module for AdCP 3.x. A new DX/feature package belongs there, so adcp/signing (the frozen copy) is untouched by this PR.

Testing

• signingtest's own tests (adcp/v3/signing/signingtest/signingtest_test.go) prove NewTestAgent + SignAndSend produce a request a real signing.Middleware-wrapped handler accepts (VerifiedSignerFromContext populated, correct algorithm), that the wired verifier is real and not a stub (rejects unsigned traffic on a RequiredFor op, rejects a replayed signature), and that SignAndSend fails fast on a non-absolute request URL (verified via a re-exec'd subprocess, since a subtest's t.Fatalf can't be observed without failing the parent test/package).
• middleware_test.go gets three new tests: ObserveOnly lets an unsigned request to a RequiredFor operation through (INFO log, no VerifiedSigner); ObserveOnly lets a well-formed-but-cryptographically-invalid signature through, with a matching ObserveOnly=false subtest proving the existing 401 behavior is unchanged; and ObserveOnly still hard-rejects a malformed Signature/Signature-Input pair (WARN log, 401, WWW-Authenticate: ... request_signature_header_malformed), proving the spec carve-out is real.
• go build ./..., go vet ./..., gofmt -l, and golangci-lint run ./signing/... are clean for every file this PR touches. The three staticcheck findings golangci-lint reports in adcp/v3/signing/jwk.go (deprecated ecdsa.PublicKey.X/.Y/.PrivateKey.D field access, Go 1.26) are pre-existing on main and untouched by this PR.
• go test ./... passes for both the adcp/v3 module (this change) and the root module (unaffected).

On item 5 of the issue ("refactor an existing boilerplate-heavy test as a demonstration")

I looked for one and came up empty, honestly reported: the request-signing profile's StaticJWKSResolver/NewMemoryReplayStore boilerplate pattern this issue targets appears nowhere else in the repo outside adcp/signing and adcp/v3/signing themselves. adcp/v3/webhook's test files (signing_test.go, publisher_test.go, e2e_test.go) use the webhook-signing profile via a different, already-existing dedicated helper (webhookKeypair) and a different verifier entry point (webhook.HTTPHandler, not signing.Middleware) — not a good refactor target for signingtest, which is scoped to the request-signing/signing.Middleware path the issue describes.

I also couldn't refactor adcp/v3/signing/middleware_test.go's own TestMiddlewareEndToEndSignAndVerify (the literal ~30-line pattern quoted in the issue) in place: that file is package signing (internal, white-box), and signingtest imports signing — importing signingtest from inside package signing would be a cyclic import. Instead, signingtest's own TestNewTestAgentSignAndSendRoundTrip reproduces the same scenario (sign, verify via middleware, assert on VerifiedSignerFromContext) using the new two-line + one-line helpers, as the concrete demonstration of the line-count reduction, and the package's README.md/doc.go cross-reference it for discoverability.

Test plan

• cd adcp/v3 && go build ./... && go vet ./... && go test ./...
• cd adcp/v3 && golangci-lint run ./signing/...
• go build ./... && go vet ./... && go test ./... at repo root (unaffected — confirms no accidental frozen-module edits)
• git status clean in the working tree aside from the files this PR touches