Fix Gemini URL retrieval defaults and empty response errors

[ebhills]

Linked issue

Closes #1211

What changes

Gemini URL retrieval leaves temperature unset instead of forcing 0.1, allowing Google's default of 1.0. Missing or empty content now preserves the provider's finish reason and message, including TOO_MANY_TOOL_CALLS, and reports failure even when URL retrieval succeeded.

Updates the configuration documentation and adds nine regression cases.

How it was verified

• Focused caller/configuration tests: 70 passed, with credentials cleared before imports.
• scripts/test-local.ps1: 2,676 passed, 6 skipped, 139 deselected.
• Regression coverage uses SDK-parsed empty responses and checks temperature omission, successful JSON parsing, optional finish messages, and client closure.
• git diff --check passed; one commit and four files, current with main.

No live Google request or deployment was verified. Low temperature remains a suspected cause of the reported loop, consistent with Google's Gemini 3 guidance. After merge and package promotion, Eric will handle the development Lambda deployment and repeat the same URL/prompt.

Compatibility and risk

No recipe API, retry, timeout, or concurrency changes. Explicit temperature configuration remains supported. Using the provider's default can change generated output. Roll back by reverting this commit and deploying the corresponding package.

Ready-for-review checklist

• One human delivery owner is assigned (ebhills)
• The linked issue and intended milestone are correct (no milestone assigned)
• The branch is current with main and has no merge conflicts
• Focused tests pass
• New or changed behavior has direct test coverage
• Documentation/schema/configuration is updated where applicable
• The PR contains no unrelated changes
• The PR description reflects the branch's current scope and latest validation
• One primary reviewer is requested only when this PR is ready

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