Forward session and turn headers to MCP HTTP requests (#15011)

## Summary
- forward request-scoped task headers through MCP tool metadata lookups
and tool calls
- apply those headers to streamable HTTP initialize, tools/list, and
tools/call requests
- update affected rmcp/core tests for the new request_headers plumbing

## Testing
- cargo test -p codex-rmcp-client
- cargo test -p codex-core (fails on pre-existing unrelated error in
core/src/auth_env_telemetry.rs: missing websocket_connect_timeout_ms in
ModelProviderInfo initializer)
- just fix -p codex-rmcp-client
- just fix -p codex-core (hits the same unrelated auth_env_telemetry.rs
error)
- just fmt

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/rmcp-client/tests/streamable_http_recovery.rs

@@ -1,5 +1,7 @@
 use std::net::TcpListener;
 use std::path::PathBuf;
+use std::sync::Arc;
+use std::sync::Mutex as StdMutex;
 use std::time::Duration;
 use std::time::Instant;
 
@@ -77,6 +79,7 @@ async fn create_client(base_url: &str) -> anyhow::Result<RmcpClient> {
         None,
         None,
         OAuthCredentialsStoreMode::File,
+        Arc::new(StdMutex::new(None)),
     )
     .await?;