fix(core): preserve unsupported image attachments as text instead of dropping them - #41723

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Rebased onto the latest v2 — #41789 (expose local attachment paths) has landed, so the two changes now coexist in the same lowering path: supported images get the Attached file: <path> prefix, and unsupported formats (avif/tiff) get an explicit text notice that also resolves local file:// paths via the same attachmentLocation helper.

AI code review — automated review for reference; please use your judgment.

Overall: Good fix — silently swallowing an attached image is a nasty failure mode, and surfacing a named placeholder keeps the conversation honest. Suggestions:

• packages/core/src/session/runner/to-llm-message.ts L67 — the branch fires on mime.startsWith("image/") unconditionally, while the message claims "model providers do not support this format"; attachmentContent receives model, and the media() path presumably consults real per-model capability — if any configured model actually accepts e.g. image/webp/image/avif, this text would be factually wrong; tie the branch to the same capability predicate used by media() and phrase it as "this model does not support...".
• packages/core/src/session/runner/to-llm-message.ts L88-L89 — inconsistency worth a thought: unsupported non-image binaries (PDFs, audio, zip…) still fall through to return [] and vanish silently — the exact UX bug this PR fixes for images; either extend the placeholder to those types too or leave a comment explaining why images are special-cased.
• packages/core/test/session-runner-message.test.ts L475-L505 — the fallback chain attachmentLocation(file) ?? file.name ?? (source.uri …) is only tested on its middle term; add a case with a type: "uri" source and no name to pin the URI rendering (and ideally one with neither, hitting "inline attachment").
• packages/core/src/session/runner/to-llm-message.ts L88 — nit: MIME types are case-insensitive per RFC 2045, so startsWith("image/") misses IMAGE/PNG; every other comparison here is exact-match anyway, so a one-line .toLowerCase() (or documenting lowercase-normalized input) settles it.

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