fix(core): avoid duplicate image bytes in tool events

[dexhunter]

Issue for this PR

Closes #36343

Type of change

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

What does this PR do?

Production-shaped image tool output can store the same base64 bytes in both structured.content and a model-visible content[].uri. This change omits structured.content only when the value is non-empty canonical base64, both MIME types match a supported image type, and a file item contains the exact matching data URI.

Mismatched, unsupported, empty, invalid, non-media, and provider-executed compatibility data remain intact.

On the incident-shaped PNG fixture, the durable event fell from 2,007,411 to 1,003,986 bytes (49.986%), base64 occurrences fell from two to one, and chunked SSE remained parseable. This addresses local duplicate media bytes; the broader bounded-projection design remains tracked in #36444.

Supplementary autoresearch: public trajectory

How did you verify your code works?

• 48 focused session-runner tests passed
• full core suite: 1,288 passed, 6 skipped
• repository pre-push typecheck: 31/31 package tasks passed
• touched-file oxlint: 0 errors
• strict evaluator preserved replay, provider behavior, unsupported output shapes, multiple files, input immutability, output bounding, and chunked SSE parsing
• git diff --check passed

Screenshots / recordings

Not applicable; this changes a durable event projection with no UI changes.

Checklist

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

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[dexhunter]

The required issue is #36343, and it is cross-linked from the issue timeline. The needs:issue label remains because this PR targets v2, while GitHub only creates closing-issue links from keywords when a PR targets the repository default branch. A maintainer with write access may need to link it manually or remove the label.

[dexhunter]

Rebased this patch onto the current v2 branch. The rebuilt head is 50c74cef0; the patch applied without overlap with upstream changes. Fresh validation on this head: 19 focused tests passed, the 38-package workspace typecheck passed, touched-file oxlint reported 0 errors, and git diff --check passed.