[codex-analytics] rework thread_source for thread analytics

[rhan-oai]

Summary

• make thread_source an explicit optional thread-level field on thread/start, thread/fork, and returned thread payloads
• persist thread_source in rollout/session metadata so resumed live threads retain the original value
• replace the old best-effort session_source -> thread_source mapping with an explicit caller-supplied analytics classification

Why

Before this change, analytics thread_source was populated by a best-effort mapping from session_source. session_source describes the runtime/client surface, not the actual thread-level origin, so that projection was not accurate enough to distinguish cases such as user, subagent, memory_consolidation, and future thread origins reliably.

Making thread_source explicit keeps one thread-level analytics field while letting callers provide the real classification directly instead of recovering it indirectly from session_source.

Impact

For new analytics events, thread_source now reflects the explicit thread-level classification supplied by the caller rather than an inferred value derived from session_source. Existing protocol fields remain optional; callers that omit threadSource now produce null instead of a best-effort inferred value.

Validation

• just write-app-server-schema
• cargo test -p codex-analytics -p codex-core -p codex-app-server-protocol --no-run
• cargo test -p codex-app-server-protocol generated_ts_optional_nullable_fields_only_in_params
• cargo test -p codex-analytics thread_initialized_event_serializes_expected_shape
• cargo test -p codex-core resume_stopped_thread_from_rollout_preserves_thread_source