Fix Codex client detection without user-agent

[joeldickson]

Summary

• Cache initialized MCP client identity by peer when no session id is available yet
• Promote peer-derived identity into the session cache once a later request provides the MCP session id
• Add regression coverage for codex-mcp-client with missing user-agent and bump version to 0.12.0

Verification

• .venv/bin/ruff check src tests
• .venv/bin/pytest tests/test_telemetry.py
• .venv/bin/pytest

Closes #27