Expose session sources to MCP contributors (#37167)

## What changed

- Add `session_source()` to `McpServerContributionContext` for thread-scoped MCP resolution.
- Propagate each thread's `SessionSource` through initial setup, per-step resolution, and runtime refreshes.

## Testing

- Verify global contexts omit the source while thread-scoped contexts retain it.
- Verify MCP contributors observe distinct sources for a root thread and its spawned subagent.

GitOrigin-RevId: 23da1509858e023fbd9e2d179034ddc83553f17b

codex-rs/core/src/session/mod.rs

@@ -304,6 +304,7 @@ use crate::exec_policy::ExecPolicyUpdateError;
 use crate::guardian::GuardianReviewOptions;
 use crate::guardian::GuardianReviewSessionManager;
 use crate::mcp::McpManager;
+use crate::mcp::McpThreadIdentity;
 use crate::network_policy_decision::execpolicy_network_rule_amendment;
 use crate::rollout::map_session_init_error;
 use crate::session_startup_prewarm::SessionStartupPrewarmHandle;