Support images and PDFs from tools and sub-agents

[lukebuehler]

Tools previously left images and PDFs as stored output or rejected them as non-text files, so models could not inspect or reference them. This change delivers admitted media through native provider content and lets models link it in replies with stable media: handles.

Changes

• Add bounded media admission and companion context entries for native MCP results and read_file on both VFS and execution-environment filesystems. Supported images and PDFs are stored in CAS; unsupported or excess media produces an omission note or tool error.
• Announce content-derived handles in Anthropic Messages, OpenAI Responses, and OpenAI Chat Completions. Preserve provider-required tool-result ordering and replace tool media with a note for the text-only DeepSeek dialect.
• Carry media linked in a sub-agent's final answer into its result envelope, retaining the referenced blobs. Runtime activities prepare supplemental context for awaited and joined results; the deterministic engine validates and sequences those entries after the corresponding tool result.
• Expose media descriptors in the API and generated TypeScript types. Render tool thumbnails, document links, and assistant media: links in the transcript, using browser object URLs for clickable previews with per-view cleanup and loader-scoped fetch caching.
• Document the capability, provider caveats, and accepted limits; add unit and opt-in live coverage.

Validation

• cargo test -p engine -p temporal-workflow -p tools -p llm-runtime -p api -p api-projection -p temporal-server --lib --quiet — 1,172 passed, 1 ignored.
• npm run test --workspace @lightspeed/platform-web -- src/components/session/media.test.tsx src/lib/sessions/transcript.test.ts src/pages/SessionsPage.test.tsx — 85 passed.
• npm run typecheck --workspace @lightspeed/platform-web — passed.
• git diff --check origin/main...HEAD — passed.

Live/credentialed suites were not run during PR preparation.

Deployment and accepted limits

The await/join resume activity contract and scheduling change without a Temporal versioning patch. Existing session workflows whose histories already exercised the affected resume paths must be closed before deployment.

The web resolves handles against the loaded transcript; a reference whose originating media is outside that loaded history may remain unresolved. Sub-agent hand-off resolves links against the child's active context, so media compacted out before its final answer is not handed up. A session-wide media lookup is deferred.