[rmcp-client] Recover from streamable HTTP 404 sessions

[caseychow-oai]

Summary

• add one-time session recovery in RmcpClient for streamable HTTP MCP 404 session expiry
• rebuild the transport and retry the failed operation once after reinitializing the client state
• extend the test server and integration coverage for 404, 401, single-retry, and non-session failure scenarios

Testing

• just fmt
• cargo test -p codex-rmcp-client (the post-rebase run lost its final summary in the terminal; the suite had passed earlier before the rebase)
• just fix -p codex-rmcp-client

[caseychow-oai]

This change is part of the following stack:

• [rmcp-client] Recover from streamable HTTP 404 sessions #13514 ◀

_{Change managed by git-spice.}