fix(core): degrade malformed tool-result content instead of crashing prepare - #50776

Issue for this PR

Closes #50775

Type of change

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

What does this PR do?

unsupportedParts and boundImages assume every tool-result with result.type === "content" has an array value. When one shows up without it, the .map throws out of prepare and the whole session drain dies with it, leaving the session permanently stuck (every subsequent prompt hits the same part).

This replaces such parts with a short text placeholder instead of throwing, and skips them in the image-byte accounting.

One thing worth calling out: just passing the part through doesn't work here, because Message.make re-validates the message right after and throws on the schema violation anyway. Swapping in a valid text result is the only way I found that actually lets the turn continue.

How did you verify your code works?

Added two tests in packages/core/test/session-model-request.test.ts feeding a malformed tool-result part (content result with value: undefined) through both functions; they used to throw TypeError: part.result.value.map is not a function, now they degrade to the placeholder. bun test test/session-model-request.test.ts passes (8/8, existing tests unaffected).

Screenshots / recordings

Not a UI change.

Checklist

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