test: add regression test for issue #196 (Anthropic 1-hour cache TTL)

[robert-j-y]

Description

Adds a regression test for issue #196 following the pattern established in PR #369.

The test uses the exact code structure from the issue report:

• System message + user message with 3 text parts
• Uses cacheControl (camelCase) exactly as the user reported
• Includes ttl: '1h' parameter for 1-hour caching

The test verifies:

• The TTL parameter is accepted by the OpenRouter API without errors
• Cache is populated on first call and hit on subsequent calls (cachedTokens > 0)

Updates since last revision

• Removed root cause analysis from header comment (keeping only known facts per reviewer feedback)
• Removed redundant inline comments throughout the test file

Human Review Checklist

• Test structure matches the exact code pattern from issue 1 hour prompt caching problem #196 (system message + 3 text parts with cacheControl camelCase)
• Cache hit verification (cachedTokens > 0) is appropriate and not flaky
• Header comment describes only known facts (no speculative root cause analysis)

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 (empty changeset for test-only change)

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Requested by: Robert Yeakel (@robert-j-y)
Link to Devin run: https://app.devin.ai/sessions/2d9573034b76446baf55558962afa10f