test: add regression test for issue #181 (tool response for image)

[robert-j-y]

Description

Adds a regression test for issue #181 ("tool response for image not working") following the e2e/issues/ pattern established in PR #369.

Test coverage (5 test cases):

• Tool result with content type containing base64 image data (file-data)
• Tool result with content type containing image URL (file-url)
• Tool result with file-data type reproducing issue tool response for image not working #181 scenario (AI SDK v3 format)
• Text-only tool results (baseline)
• JSON tool results (baseline)

The test documents the observed behavior: tool results with multimodal content (images) are JSON.stringified.

Updates since last revision

• Removed root cause analysis from test comments per review feedback - test now only documents known facts (reported behavior and observed behavior)
• Uses AI SDK v3 format (type: "file-data") instead of V2 type: "media" format from the original issue

Human Review Checklist

• Verify the test accurately reproduces the issue tool response for image not working #181 scenario
• Confirm the test assertions correctly document the stringification behavior
• Note: This is a unit test placed in e2e/issues/ to follow the issue regression test pattern

Checklist

• I have run pnpm stylecheck and pnpm typecheck
• I have run pnpm test and all tests pass
• I have added tests for my changes (if applicable)
• I have updated documentation (if applicable)

Changeset

• I have run pnpm changeset to create a changeset file

Note: Empty changeset used since this is a test-only change that doesn't need a release.

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Link to Devin run: https://app.devin.ai/sessions/b5fe98b56d854923975d83d933cdbf45
Requested by: Robert Yeakel (@robert-j-y)