feat(core): describe images with a vision model when the session model cannot see them - #62534

Issue for this PR

Ports #26164 to V2. That PR (and its issue #26160) were closed as V1-only with the ask to port the feature to V2.

Type of change

• New feature

What does this PR do?

When the session model cannot see images, V2 currently replaces every image with ERROR: Cannot read "x.png" (this model does not support image input). Inform the user. (via unsupportedParts in SessionModelRequest). This PR instead describes the image once with a vision-capable model and substitutes that description, so the conversation can continue on a text-only model.

• New SessionImageDescribe service runs inside SessionModelRequest.prepare, before unsupportedParts: it replaces unsupported image media (user attachments, assistant media, and tool-result images with a local payload) with [Image "name" converted to text by provider/model ...] + description. Images the describer cannot serve keep the existing placeholder, so behavior only improves.
• Descriptions are generated once per image per describing model (in-process cache keyed by content hash) and reused by every later step, compaction, and title request.
• Describing model resolution: experimental.image_model in config → automatic. Automatic selection is the new Model.image(providerID?), which prefers the session provider's own vision models, then a known cheap vision family anywhere — it never silently picks a stranger's flagship by release order.
• Nothing is sent when the model already sees images, when no describer is available, or when a description call fails — all degrade to today's placeholder.

Gated behind the image_fallback feature flag (on in local/dev builds, off in releases; OPENCODE_FLAGS=image_fallback or experimental.image_fallback: true enable it elsewhere). The V1 PR's TUI model picker is intentionally not ported — V2 has no per-prompt model plumbing, so the describer is selected through config; a picker can follow if this lands.

How did you verify your code works?

• New packages/core/test/session-image-describe.test.ts (9 tests): description substitution for user media and tool-result images, request shape (describe prompt, filename, user-message context), per-image caching, passthrough when the model sees images / no describer exists / the flag is off / the describe call fails, configured experimental.image_model preference, and Model.image catalog selection against the real Model layer.
• Existing session-model-request* and provider-plugin suites pass (they compose SessionModelRequest.layer with a passthrough SessionImageDescribe).
• bun run check green; client types and openapi.json regenerated with their scripts.

Visual

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Checklist

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