[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-protocol/src/protocol/common.rs

@@ -2184,6 +2184,7 @@ mod tests {
                     cwd: cwd.clone(),
                     cli_version: "0.0.0".to_string(),
                     source: v2::SessionSource::Exec,
+                    thread_source: None,
                     agent_nickname: None,
                     agent_role: None,
                     git_info: None,
@@ -2226,6 +2227,7 @@ mod tests {
                         "cwd": absolute_path_string("tmp"),
                         "cliVersion": "0.0.0",
                         "source": "exec",
+                        "threadSource": null,
                         "agentNickname": null,
                         "agentRole": null,
                         "gitInfo": null,