[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/app-server/src/request_processors/thread_processor.rs

@@ -746,6 +746,7 @@ impl ThreadRequestProcessor {
             personality,
             ephemeral,
             session_start_source,
+            thread_source,
             environments,
             persist_extended_history,
         } = params;
@@ -799,6 +800,7 @@ impl ThreadRequestProcessor {
                 typesafe_overrides,
                 dynamic_tools,
                 session_start_source,
+                thread_source.map(Into::into),
                 environment_selections,
                 service_name,
                 experimental_raw_events,
@@ -882,6 +884,7 @@ impl ThreadRequestProcessor {
         typesafe_overrides: ConfigOverrides,
         dynamic_tools: Option<Vec<ApiDynamicToolSpec>>,
         session_start_source: Option<codex_app_server_protocol::ThreadStartSource>,
+        thread_source: Option<codex_protocol::protocol::ThreadSource>,
         environments: Option<Vec<TurnEnvironmentSelection>>,
         service_name: Option<String>,
         experimental_raw_events: bool,
@@ -998,6 +1001,7 @@ impl ThreadRequestProcessor {
                     codex_app_server_protocol::ThreadStartSource::Clear => InitialHistory::Cleared,
                 },
                 session_source: None,
+                thread_source,
                 dynamic_tools: core_dynamic_tools,
                 persist_extended_history: false,
                 metrics_service_name: service_name,
@@ -2382,6 +2386,11 @@ impl ThreadRequestProcessor {
                         return Ok(());
                     }
                 };
+                thread.thread_source = codex_thread
+                    .config_snapshot()
+                    .await
+                    .thread_source
+                    .map(Into::into);
 
                 self.thread_watch_manager
                     .upsert_thread(thread.clone())
@@ -2869,6 +2878,7 @@ impl ThreadRequestProcessor {
             base_instructions,
             developer_instructions,
             ephemeral,
+            thread_source,
             exclude_turns,
             persist_extended_history,
         } = params;
@@ -2959,6 +2969,7 @@ impl ThreadRequestProcessor {
                     history: history_items.clone(),
                     rollout_path: source_thread.rollout_path.clone(),
                 }),
+                thread_source.map(Into::into),
                 /*persist_extended_history*/ false,
                 self.request_trace_context(&request_id).await,
             )
@@ -3018,6 +3029,11 @@ impl ThreadRequestProcessor {
             }
             thread
         };
+        thread.thread_source = forked_thread
+            .config_snapshot()
+            .await
+            .thread_source
+            .map(Into::into);
 
         self.thread_watch_manager
             .upsert_thread_silently(thread.clone())
@@ -3620,6 +3636,7 @@ pub(crate) fn thread_from_stored_thread(
         agent_nickname: source.get_nickname(),
         agent_role: source.get_agent_role(),
         source: source.into(),
+        thread_source: thread.thread_source.map(Into::into),
         git_info,
         name: thread.name,
         turns: Vec::new(),
@@ -3682,6 +3699,7 @@ fn summary_from_state_db_metadata(
     cwd: PathBuf,
     cli_version: String,
     source: String,
+    _thread_source: Option<codex_protocol::protocol::ThreadSource>,
     agent_nickname: Option<String>,
     agent_role: Option<String>,
     git_sha: Option<String>,
@@ -3732,6 +3750,7 @@ fn summary_from_thread_metadata(metadata: &ThreadMetadata) -> ConversationSummar
         metadata.cwd.clone(),
         metadata.cli_version.clone(),
         metadata.source.clone(),
+        metadata.thread_source,
         metadata.agent_nickname.clone(),
         metadata.agent_role.clone(),
         metadata.git_sha.clone(),
@@ -3815,6 +3834,7 @@ fn build_thread_from_snapshot(
         agent_nickname: config_snapshot.session_source.get_nickname(),
         agent_role: config_snapshot.session_source.get_agent_role(),
         source: config_snapshot.session_source.clone().into(),
+        thread_source: config_snapshot.thread_source.map(Into::into),
         git_info: None,
         name: None,
         turns: Vec::new(),