fix(sidebar): refresh model after session switches

[Usernames686]

Summary

• refresh the persisted sidebar model projection when the transcript reports a different model
• keep unchanged detail reads write-free and preserve conversation recency by leaving updated_at untouched
• reuse the existing conversation upsert path so sidebar metadata converges after an in-session model switch

Tests

• cargo test --features test-utils db::service::conversation_service::tests --lib
• cargo check
• cargo check --no-default-features --bin codeg-server
• cargo check --no-default-features --bin codeg-mcp
• cargo clippy --all-targets --features test-utils -- -D warnings
• cargo clippy --no-default-features --bin codeg-server --lib -- -D warnings
• cargo clippy --no-default-features --bin codeg-mcp -- -D warnings
• cargo test --features test-utils
• cargo test --no-default-features --bin codeg-server --lib