[codex-analytics] rework thread_source for thread analytics (#20949)

## 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`

codex-rs/thread-store/src/local/create_thread.rs

@@ -34,6 +34,7 @@ pub(super) async fn create_thread(
             params.thread_id,
             params.forked_from_id,
             params.source,
+            params.thread_source,
             params.base_instructions,
             params.dynamic_tools,
             event_persistence_mode(params.event_persistence_mode),

codex-rs/thread-store/src/local/helpers.rs

@@ -130,6 +130,7 @@ pub(super) fn stored_thread_from_rollout_item(
         cwd: item.cwd.unwrap_or_default(),
         cli_version: item.cli_version.unwrap_or_default(),
         source,
+        thread_source: None,
         agent_nickname: item.agent_nickname,
         agent_role: item.agent_role,
         agent_path: None,

codex-rs/thread-store/src/local/mod.rs

@@ -744,6 +744,7 @@ mod tests {
             thread_id,
             forked_from_id: None,
             source: SessionSource::Exec,
+            thread_source: None,
             base_instructions: BaseInstructions::default(),
             dynamic_tools: Vec::new(),
             metadata: thread_metadata(),

codex-rs/thread-store/src/local/read_thread.rs

@@ -274,10 +274,11 @@ async fn stored_thread_from_sqlite_metadata(
             .ok()
             .flatten(),
     };
-    let forked_from_id = read_session_meta_line(metadata.rollout_path.as_path())
+    let session_meta = read_session_meta_line(metadata.rollout_path.as_path())
         .await
         .ok()
-        .and_then(|meta_line| meta_line.meta.forked_from_id);
+        .map(|meta_line| meta_line.meta);
+    let forked_from_id = session_meta.as_ref().and_then(|meta| meta.forked_from_id);
     StoredThread {
         thread_id: metadata.id,
         rollout_path: Some(metadata.rollout_path),
@@ -297,6 +298,7 @@ async fn stored_thread_from_sqlite_metadata(
         cwd: metadata.cwd,
         cli_version: metadata.cli_version,
         source: parse_session_source(&metadata.source),
+        thread_source: metadata.thread_source,
         agent_nickname: metadata.agent_nickname,
         agent_role: metadata.agent_role,
         agent_path: metadata.agent_path,
@@ -362,6 +364,7 @@ fn stored_thread_from_meta_line(
         cwd: meta_line.meta.cwd,
         cli_version: meta_line.meta.cli_version,
         source: meta_line.meta.source,
+        thread_source: meta_line.meta.thread_source,
         agent_nickname: meta_line.meta.agent_nickname,
         agent_role: meta_line.meta.agent_role,
         agent_path: meta_line.meta.agent_path,