feat(bqaa): ADK 2.0 minimum producer cut for google#293 v5

Implements the customer-driven mid-June producer-only subset of the
ADK 2.0 observability work tracked in google#293 (parent google#190). The customer
needs these specific fields visible in BigQuery before their ADK 2.0
production cutover; full v15 contract (google#190) lands incrementally.

This change is producer-only. No consumer-side SDK / typed-view work
is included — the customer reads base-table JSON directly during the
mid-June window.

What lands
----------

A1/A2 — every ADK-enriched row carries the attributes.adk envelope:
  attributes.adk.schema_version (_ADK_ENVELOPE_SCHEMA_VERSION = "1")
  attributes.adk.app_name       (from InvocationContext.session.app_name)

A3 — rows from on_event_callback additionally carry:
  attributes.adk.source_event_id  (Event.id, reliable join key)
                                  Note: never fabricated — callback rows
                                  without an originating Event leave it
                                  JSON-null.

C1 — attributes.adk.node = {path, run_id, parent_path}. parent_path is
     derived from path; default-empty path (NodeInfo.path = "") is
     preserved verbatim with parent_path = null, no synthesis.

C2 — attributes.adk.branch (absent stays JSON null).

C3 — attributes.adk.scope = null | {id, kind} per google#198 / google#293 v5
     derivation order: (1) None → null, (2) name@run_id / path/name@run_id
     → node_run, (3) any other non-empty string → function_call
     (model-provided FC IDs match here), (4) empty/non-string → unknown
     with warning.

C4 — emit AGENT_TRANSFER from event.actions.transfer_to_agent. Payload
     pinned: from_agent = event.author, to_agent = the target. Verified
     against EventActions.transfer_to_agent which stores the target only.

C5 — emit EVENT_COMPACTION from event.actions.compaction. Float
     start_timestamp / end_timestamp preserved with fractional precision
     (consumer view conversion deferred).

C6 — emit AGENT_STATE_CHECKPOINT when actions.agent_state is not None
     OR actions.end_of_agent is True. Allows {agent_state: null,
     end_of_agent: true} payloads. Inline payload only; GCS offload
     for oversized state deferred.

C7 — emit TOOL_PAUSED for each event.long_running_tool_ids id, with
     attributes.pause_kind (derived from function_call NAME via
     _HITL_PAUSE_KIND_MAP — hitl_* for adk_request_*, tool otherwise)
     and attributes.function_call_id. HITL routing is unchanged:
     HITL function_responses stay on HITL_*_COMPLETED, NEVER emit
     TOOL_COMPLETED. Non-HITL function_responses arriving via
     on_user_message_callback emit TOOL_COMPLETED with pause_kind='tool'
     so the customer can pair (TOOL_PAUSED ↔ TOOL_COMPLETED) on
     (app_name, user_id, session_id, function_call_id) directly in SQL.
     Pause registry / pause_orphan semantics deferred to google#206.

C8 — attributes.adk.{route, render_ui_widgets, rewind_before_invocation_id}
     mirror EventActions, flat-with-prefix per google#203 (matches the rest
     of the attributes.adk.* envelope convention).

D1 — delete the deprecated on_state_change_callback stub (never called
     by ADK 2.0; verified no callers).

Compatibility
-------------

* AGENT_RESPONSE retains the legacy flat extras (source_event_id,
  source_event_author, source_event_branch) for backward compat. The
  canonical keys are now under attributes.adk.*.
* The HITL test fixtures use Mock events without long_running_tool_ids
  or .id; the envelope helper is defensive against missing attrs.
* No EventData / _log_event signature change. Added one optional field
  EventData.source_event: Optional[Event] = None — a minimal B0 (google#194)
  step. Callbacks that have access to the source Event pass it through;
  others leave it None (and the envelope correctly leaves A3/C1/C2/C3
  null on those rows).

Tests
-----

257 plugin tests pass (238 existing + 19 new):
* envelope shape on event-originating and non-event-originating rows
* node parent_path derivation with both empty and nested paths
* _derive_scope for None, bare node, path/node, FC IDs, empty string
* C4/C5/C6 emit paths
* C5 fractional float-epoch precision round-trip
* C6 both-shape coverage + id-stabilization regression guard
  (Event.model_post_init auto-assigns id even when constructor omits it)
* C7 TOOL_PAUSED pause_kind derivation for non-HITL and HITL
* C7 HITL non-routing: HITL function_response → HITL_*_COMPLETED only,
  NEVER TOOL_COMPLETED
* C7 user-message TOOL_COMPLETED with pause_kind='tool'
* C8 flat-with-prefix route / rewind_before_invocation_id
* D1: on_state_change_callback removed from the public surface

Refs: google#293 (v5), google#190, google#194, google#196, google#197, google#198, google#199, google#200, google#201, google#202,

Author: caohy1988