fix(hosting): export BYO_BRAND so the public ByoValue type resolves - #3336

Problem

check_api_changes is currently failing on main (and therefore on every PR built on it) with a TypeScript compile error in @aws-amplify/hosting:

Error: Validation of @aws-amplify/hosting failed, compiler output:
index.ts: error TS2304: Cannot find name 'BYO_BRAND'.
index.ts: error TS2741: Property '[BYO_BRAND]' is missing in type 'ByoValueBaseline' but required in type 'ByoValue'.

@aws-amplify/hosting exports the ByoValue type, whose shape is { readonly [BYO_BRAND]: true; ... }. But BYO_BRAND is a module-private const in packages/hosting/src/byo.ts (const BYO_BRAND = Symbol.for(...), not exported). When check_api_changes compiles the public API surface, the exported ByoValue references a name that isn't exported alongside it, so the compile fails. This blocks the check on main and on every open PR that includes the hosting package (e.g. #3326, #3329).

Issue number, if available: N/A (CI/tooling fix; main is red on check_api_changes after the standalone-SSR-hosting change)

Changes

• packages/hosting/src/byo.ts: export the BYO_BRAND symbol so the exported ByoValue type's shape is fully resolvable during API extraction. Runtime behavior is unchanged — it remains the same Symbol.for('@aws-amplify/hosting.byo') registry symbol (the comment already notes Symbol.for is chosen so it survives across module copies).
• Adds a patch changeset for @aws-amplify/hosting.

No API.md change: api-extractor renders the [BYO_BRAND] computed key in ByoValue the same way whether or not the symbol is exported, so exporting it resolves the compile error with zero change to the emitted public-API surface (verified by regenerating packages/hosting/API.md locally — no diff).

Corresponding docs PR, if applicable: N/A

Validation

• Built @aws-amplify/hosting locally with @aws-blocks/hosting installed: tsc --build packages/hosting exits 0 (previously failed on the BYO_BRAND reference). The emitted packages/hosting/lib/byo.d.ts now contains export declare const BYO_BRAND: unique symbol;, so the exported ByoValue type resolves.
• Regenerated packages/hosting/API.md via api-extractor run --local: no change (confirms zero public-API-surface delta).
• Manual verification: N/A — this is a compile-resolution fix in a build script's input; the check_api_changes CI step is the exercising path.

Checklist

• If this PR includes a functional change to the runtime behavior of the code, I have added or updated automated test coverage for this change.
• If this PR requires a change to the Project Architecture README, I have included that update in this PR.
• If this PR requires a docs update, I have linked to that docs PR above.
• If this PR modifies E2E tests, makes changes to resource provisioning, or makes SDK calls, I have run the PR checks with the run-e2e label set.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

no linked-issue close: CI/tooling fix unblocking check_api_changes on main; not tied to a tracked issue.