Support provider User-Agent overrides

[mipsel64]

Summary

• add an optional user_agent setting to OpenAI, OpenRouter, and Z.ai providers
• validate configured values at startup and apply them only to upstream inference requests
• document and test a Codex-shaped User-Agent while preserving tinyllm/<version> by default
• upgrade rustls to 0.23.45 to address RUSTSEC-2026-0285

Testing

• cargo fmt --all -- --check
• cargo clippy --locked --all-targets -- -D warnings
• cargo test --locked --all-targets
• git diff --check