feat(streaming): tag streamed events with agent_path

[vkalmathscale]

Why

Streamed lifecycle and text events carry only a task_id. When multiple agents share one task stream (for example, child-workflow sub-agents emitting into a parent orchestrator's stream), their events are indistinguishable at the consumer. The only identity-ish field is author (Literal["user","agent"]), a role, not a source.

What

Thread an optional agent_path (str | list[str]), a single agent id or a root-to-emitter path like ["researcher", "subagent-abc"], end to end:

• Wire: TaskMessage.agent_path on the envelope, stamped on the in-memory message after messages.create(), so every start/delta/full/done event carries it via parent_task_message. No change to the messages.create/update client contract (stream-only tag).
• Propagation: ContextInterceptor gains a streaming_agent_path ContextVar plus a context-agent-path header, threaded from workflow to activity independently of the task/trace/span gate (a child workflow can set only _agent_path). This lets the offloaded model activity tag live text and reasoning deltas.
• Emission: TemporalStreamingHooks takes agent_path and forwards it on tool-request, tool-response, and handoff; stream_lifecycle_content and the streaming service/facade pass it to the envelope; TemporalStreamingModel reads the ContextVar for its three streaming contexts.

Backward compatible

agent_path defaults to None (untagged) everywhere. Existing callers, including subclasses that call super().__init__(task_id=..., timeout=...), are unaffected.

Depends on scaleapi/scale-agentex#381 (do not merge standalone)

The TaskMessage.agent_path wire field is authored upstream in the backend OpenAPI spec (scaleapi/scale-agentex#381). The edit to src/agentex/types/task_message.py in this PR is a temporary manual patch so the branch is functional and testable now; merging a hand-edited generated type would be wiped on the next Stainless regen.

Merge order:

1. Merge scale-agentex#381 (backend schema plus regenerated openapi.yaml).
2. Stainless auto-regenerates this SDK on next, adding agent_path to the generated types/task_message.py.
3. This PR is rebased to drop the manual types/task_message.py patch, leaving only the lib/ runtime.

Verification

• py_compile clean on all touched files; lint (ruff/FA102) fixed.
• Runtime: TaskMessage.agent_path serializes correctly for str, list[str], and the None default via model_dump(mode="json").
• Deeper Temporal-runtime checks (header round-trip, replay) should run in a bootstrapped env.

Follow-ups (not in this PR)

• Consumers read parent_task_message.agent_path through a single toPath(x) = Array.isArray(x) ? x : [x] normalizer (for the str | list union).
• Multi-agent consumers with nested sub-agents adopt agent_path (the parent sets each child's path) and retire any bespoke per-agent attribution workaround.

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