fix(startup): downgrade LLM verify_connection failure to warning

[nicoloboschi]

Summary

• When the LLM provider is unavailable at startup (e.g. 429 quota exhaustion), the server now logs a warning and continues booting instead of crash-looping
• Each distinct LLM config (default, retain, reflect, consolidation) is verified independently — a failure on one doesn't skip the others
• Adds unit tests using mock provider to verify the soft-fail and happy-path behavior

Fixes #1147

Test plan

• test_initialize_succeeds_when_verify_connection_fails — verify_connection raises, server still boots, warning logged
• test_initialize_logs_warning_per_failing_config — two distinct configs fail, two separate warnings logged with config names
• test_initialize_succeeds_when_verify_connection_succeeds — happy path, no warnings