Add unit tests for custom model endpoint with Anthropic thinking blocks

[okottorika]

Problem

The existing tests for issue #146 (custom Anthropic models with ThinkingLevel enabled) were integration tests requiring a real ANTHROPIC_API_KEY, making them impossible to run in CI or for most contributors.

Solution

This PR adds two unit tests using a mock SSE server to verify the custom model test endpoint behavior:

Tests Added

1. TestCustomModelTestEndpointWithMockAnthropic — Verifies that responses with thinking blocks followed by text content are handled correctly. The mock server returns a realistic Anthropic SSE stream with a thinking block followed by a text block, confirming the endpoint correctly skips thinking blocks and finds the text response.

2. TestCustomModelTestEndpointOnlyThinkingBlocks — Verifies that responses containing ONLY thinking blocks (no text) are properly rejected with the "empty response from model" error. This ensures the validation logic doesn't false-positive on thinking-only responses.

Context

Issue #146 was previously fixed in commit f01b163, which updated the response validation logic to iterate through all content blocks instead of only checking the first one. However, no test coverage existed without a real API key. These tests provide that coverage using a mock HTTP server.

Testing

• All custom model tests pass locally
• Tests run without requiring ANTHROPIC_API_KEY
• No changes to production code — test coverage only

Fixes #146