feat: Add schema auto-upgrade, tool provenance, HITL tracing, and span hierarchy fix to BigQuery Agent Analytics plugin

This CL adds four enhancements to the BigQuery Agent Analytics plugin and fixes a span hierarchy corruption bug.

- **Schema Auto-Upgrade:** Additive-only schema migration that automatically adds missing columns to existing BQ tables on startup. A `adk_schema_version` label on the table (starting at `"1"`, bumped with each schema change) makes the check idempotent — the diff runs at most once per version. Enabled by default (`auto_schema_upgrade=True`) because upgrades are additive-only and fail-safe. Pre-versioning tables (no label) are treated as outdated, diffed, and stamped. No previous schema versions need to be stored; the logic diffs actual columns against the current canonical schema.

- **Tool Provenance:** Adds `tool_origin` to TOOL_* event content, distinguishing six origin types — `LOCAL` (FunctionTool), `MCP` (McpTool), `A2A` (AgentTool wrapping RemoteA2aAgent), `SUB_AGENT` (AgentTool), `TRANSFER_AGENT` (TransferToAgentTool), and `UNKNOWN` (fallback) — via `isinstance()` checks with lazy imports to avoid circular dependencies.

- **HITL Tracing:** Emits dedicated HITL event types (`HITL_CONFIRMATION_REQUEST`, `HITL_CREDENTIAL_REQUEST`, `HITL_INPUT_REQUEST` + `_COMPLETED` variants) for human-in-the-loop interactions. Detection lives in `on_event_callback` (for synthetic `adk_request_*` FunctionCall events emitted by the framework) and `on_user_message_callback` (for `adk_request_*` FunctionResponse completions sent by the user), not in tool callbacks — because `adk_request_*` names are synthetic function calls that bypass `before_tool_callback`/`after_tool_callback` entirely.

- **Span Hierarchy Fix (#4561):** Removes `context.attach()`/`context.detach()` calls from `TraceManager.push_span()`, `attach_current_span()`, and `pop_span()`. The plugin was injecting its spans into the shared OTel context, which corrupted the framework's span hierarchy when an external exporter (e.g. `opentelemetry-instrumentation-vertexai`) was active — causing `call_llm` to be re-parented under `llm_request` and parent spans to show shorter durations than children. The plugin now tracks span_id/parent_span_id via its internal contextvar stack without mutating ambient OTel context.

Co-authored-by: Haiyuan Cao <haiyuan@google.com>
PiperOrigin-RevId: 873114688

Author: haiyuan-eng-google