fix(ai): normalize tool-result image data URLs - #1421

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().

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

A focused converter regression is included and the branch is based directly on current upstream main (5a23f6edfa628f5d1a86422f683742e092feebc2). Repository CI is expected to exercise the package test/typecheck and changelog gates.

Closes #1260

Summary by cubic

Fixes tool-result images with already-prefixed data URLs getting double-prefixed in Responses serializers, producing invalid image_url values. Now strips the canonical data:<mime>;base64, prefix from tool-result image data before provider serialization so only one prefix gets added.

• Only strips prefixes that exactly match the block's declared MIME type; raw base64 and mismatched data: payloads pass through unchanged.
• Adds regression tests covering raw base64 and already-prefixed image data through convertResponsesMessages().
• Closes Tool-result image data URLs are double-prefixed for Codex Responses, producing invalid image_url #1260.

^{Written for commit 9b082b4. Summary will update on new commits.}