feat(core): promote managed attachment media on first turn

[safzanpirani]

Issue for this PR

Closes #46173 (PR 2 of the 3-PR plan there; stacked on #46175 — only the top commit f9e6c06c6 is this PR's diff, the rest is #46175's base)

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Native media delivery for managed attachments. On the first provider turn that consumes a prompt, a stored image or PDF is sent as native media (path in the media part's filename) when the model's catalog capabilities, the sniffed MIME, the protocol route, and the decoded-size limit all allow it. Anything disqualified degrades to the path-only text part from #46175 instead of aborting. Later turns replay path-only — the file is never re-read or re-encoded, and no base64 is persisted (the #42263 failure class). Delivery is at-most-once via a durable nativeMediaDeliveredAt marker written before provider I/O. Media admission is owned by each protocol (Chat Completions: images only; Responses/Anthropic/Gemini: images + PDF), so core can't drift from what lowering accepts.

How did you verify your code works?

• packages/core: focused suites 124 pass — includes a two-real-runner-turn test with a recording LLM client that kills and reopens the runtime/store between turns, then asserts the replay request is path-only and raw session_message rows contain no media base64. Typecheck clean.
• packages/llm: 300 pass / 30 skipped, typecheck clean — per-protocol admission and PDF lowering tests for Responses, Anthropic, Gemini; Chat Completions PDF rejection.
• packages/opencode: HTTP attachment suite 5 pass.

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search results, I found one related PR that is not a duplicate but rather a stacked dependency:

Related PR (not a duplicate):

• PR feat(core): materialize managed attachments for v2 sessions #46175: feat(core): materialize managed attachments for v2 sessions
  • feat(core): materialize managed attachments for v2 sessions #46175
  • This is the base PR that PR feat(core): promote managed attachment media on first turn #46182 is stacked on (as mentioned in the description: "stacked on feat(core): materialize managed attachments for v2 sessions #46175")

This is intentional as per the PR description which states this is "PR 2 of the 3-PR plan" and only the top commit represents the current PR's diff, with the rest being #46175's base.

The searches also found PR #43314 about degrading undecodable image attachments, but this is addressing a different concern (error handling) rather than native media delivery.

No duplicate PRs found