test: add regression test for issue #248 (Gemini web search empty responses)

[robert-j-y]

Description

Adds regression tests for issue #248 (Gemini 3 Pro Preview returning empty responses when using web search plugin).

Test coverage for all models mentioned in the issue thread:

• google/gemini-3-pro-preview (original issue): generateText, streamText, and part number search queries
• google/gemini-3-flash-preview (Jan 27 comment: "works")
• anthropic/claude-sonnet-4 (Dec 7 comment: stream cuts, partial response)
• anthropic/claude-opus-4 (Dec 7 comment: bugs out, no error)
• openai/gpt-4.1 (Dec 7 comment: "only consistent experience")

Configuration: Tests use the exact code pattern from the issue report with providerOptions: { openrouter: { reasoning: { effort: 'high' }, plugins: [{ id: 'web' }] } }.

Updates since last revision

• Expanded test coverage to include all models mentioned in the issue thread (7 tests total across 5 models)
• Removed root cause speculation from comments; test header now contains only factual timeline from the issue thread
• Note: The part number search test may fail, documenting the actual current bug state

Human Review Checklist

• Verify the test code pattern matches the original issue report exactly (uses providerOptions not model-level settings)
• The part number test ("602-3973-01") covers the specific scenario from the Dec 7 comment
• Model names in tests match what was mentioned in the issue thread
• Test comments contain only facts from the issue, no speculation

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) - N/A

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/b246f84ecad04155bed9b1af8a649f56
Requested by: Robert Yeakel (@robert-j-y)