feat(test): explicit target-capability fixtures for non-text MCP content (#103 remainder)

[ScriptedAlchemy]

Summary

Closes out the target-capability acceptance item on #103: "Audio and other non-text content is projected only when the target capability fixture advertises support; fallback and fail-closed paths are both tested" — the harness previously had route-level fail-closed coverage but no dedicated capability-fixture surface in agent-bundle/test.

• createTargetCapabilityFixture() (agent-bundle/test): an immutable, explicit fixture over the issue's capability list — image, audio, resource, progress, with text as the non-configurable always-supported baseline. No rich capability defaults to supported: callers choose every boolean and the richContentFallback: 'text' | 'fail' policy, so a test can never pretend unsupported content is accepted by omission.
• projectTargetCapabilities(): reprojects a real renderRouteEvents() stream through the runtime's own projectMcpRenderStream — the same projector production uses — with exactly the advertised capabilities. Progress maps to the real progress-token/notification contract. Proof level stays honestly route-unit: real renderer, real projector, no transport/packed/host claim.
• Fail-closed: a denied kind under fallback: 'fail' surfaces as AgentTestError code unsupported-rich-content, identifying the offending kind and the fixture, with the proof level in the failure output. Under fallback: 'text' the runtime's exact placeholder blocks ([image <mime>], [audio <mime>], [resource <name> <uri>]) are asserted and no rich block leaks.
• Matchers: expectDocument() gains field-aware toContainImage / toContainAudio / toContainResource for non-text Agent Document nodes; existing text assertions are proven not to widen.
• Fixture: a route module emitting text + PNG image + WAV audio + binary (data-URI) resource nodes plus request-scoped progress, exercised across supported / fallback / fail-closed in tests/projection/target-capabilities.test.ts.

Changeset: agent-bundle minor (new public test-harness surface). No runtime behavior changes.

Gates (local)

• pnpm test:unit — 2303 passed / 5 skipped (worker run); scoped test-harness-manifest.test.ts re-run post-rebase: 32 passed
• pnpm test:route-unit — passed (post-rebase)
• pnpm test:projection — 30 passed, 0 failed (post-rebase)
• pnpm typecheck — clean
• pnpm lint — 0 errors, 0 warnings

Part of #103 (with the create-agent-bundle template-adoption PR, this completes the issue's remaining items).

[chatgpt-codex-connector]

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-01T23:38:04.484649Z	f7be27a	PR opened

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[pkg-pr-new]

npm i https://pkg.pr.new/ScriptedAlchemy/agent-bundle@239

npm i https://pkg.pr.new/ScriptedAlchemy/agent-bundle/create-agent-bundle@239

npm i https://pkg.pr.new/ScriptedAlchemy/agent-bundle/@agent-bundle/runtime@239

commit: 224c7a0

[changeset-bot]

🦋 Changeset detected

Latest commit: 224c7a0

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
agent-bundle	Minor

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