[codex] Fix Flock unit test fixtures and explicit model configs

[lintool]

Summary

• Allow explicit model configs with model and provider to resolve without a catalog row, using default secrets and defaults for optional fields.
• Seed missing test-only model and prompt fixtures in unit test setup.
• Restore expected Ollama audio transcription error coverage.

Validation

• cmake --build build/ninja-release --target flock_tests
• build/ninja-release/extension/flock/test/unit/flock_tests --gtest_filter='LLMFirstTest.AudioTranscriptionOllamaError:LLMLastTest.AudioTranscriptionOllamaError:LLMReduceTest.AudioTranscriptionOllamaError:LLMRerankTest.AudioTranscriptionOllamaError:LLMCompleteTest.LLMCompleteAudioTranscriptionError:LLMFilterTest.LLMFilterAudioTranscriptionOllamaError:ModelManagerTest.*:PromptManager.CreatePromptDetailsWithExplicitVersion:PromptManager.CreatePromptDetailsOnlyPromptName:PromptManager.CreatePromptDetailsNonExistentVersion'
• build/ninja-release/extension/flock/test/unit/flock_tests
• git diff --check

Note: clang-format was not available on PATH, so no formatter command was run.