[codex-analytics] preserve user thread source for exec threads

[marksteinbrick-oai]

Why

• Follows [codex-analytics] rework thread_source for thread analytics #20949.
• The above moved thread_source attribution from the reducer to explicit caller provided metadata
• The codex exec path still omitted this metadata, leaving exec-created threads without thread_source

What Changed

• Ensures exec threads are marked as user created (thread_source = "user")
• Preserves thread-source metadata in exec’s startup session event

Verification

• Updated unit tests to validate exec thread_source propagation.
• cargo +1.93.0 test -p codex-exec --manifest-path codex-rs/Cargo.toml
• cargo +1.93.1 build -p codex-cli --manifest-path codex-rs/Cargo.toml
• Validated locally with a freshly built codex exec run:
  • Startup logs showed thread_source: Some(User).
  • Rollout metadata recorded "thread_source":"user".