fix(ai): normalize tool-result image data URLs

[Altairpaca]

Summary

Fixes #1260 by restoring the ImageContent.data contract at the shared message-normalization boundary before provider serialization.

Tool results occasionally contain an already-prefixed data:<mime>;base64,... string even though ImageContent.data is defined as raw base64. Responses adapters then add their own prefix and produce an invalid double-prefixed image_url.

Root cause

transformMessages() normalized unsupported media and tool-call IDs but passed supported tool-result image payloads through unchanged. Provider adapters therefore had to trust that ImageContent.data was raw base64, and an already-prefixed value leaked into the Responses serializer.

Changes

• Normalize tool-result media blocks in transform-messages.ts.
• If data starts with the exact canonical prefix for its declared MIME type (data:${mimeType};base64,), strip that prefix and keep raw base64 internally.
• Leave ordinary raw base64 byte-identical.
• Do not accept/normalize arbitrary or mismatched data: payloads.
• Add deterministic regression coverage for raw base64 and already-prefixed image data through convertResponsesMessages().
• Add exact-nearest packages/ai/src/changes.md coverage for the shared normalization path.

Scope / risk

The change is limited to tool-result media normalization. User attachments are unchanged. Provider serializers retain their existing raw-base64 contract and continue adding the wire prefix themselves.

Validation / current sync

The branch is synchronized with upstream main at 464f1a54e8569cdfe8e4620583e19d594a6b96e9 (head bdc33c87e57faeb1ca34719972707ef5248fc45d). Current main still lacks tool-result data-URL normalization in transform-messages.ts, so #1260 remains live upstream. The semantic runtime diff remains exactly the production normalizer plus its focused regression; the third changed file is repository-policy tracker coverage.

PR-triggered CI, Changelog gate, and model-catalog workflows for this fork contribution may stop at action_required before jobs are created; that is an approval gate rather than executable test evidence. Any executable failure remains blocking.

Per CONTRIBUTING.md, release CHANGELOG.md entries remain maintainer-owned.

Closes #1260

[Altairpaca]

Implementation is ready for review. The branch is current-main based and the diff is limited to the shared tool-result normalization seam plus the focused #1260 regression. All three fork workflows are currently action_required with no jobs executed; when convenient, please approve the workflow run. I’ll own any CI/review follow-up through merge.