Apply Google ADK deterministic providers inside workflow tasks

[DABH]

ADK keeps its time and uuid providers in contextvars.ContextVars, and GoogleAdkPlugin set them from the worker's context. Workflow tasks run on the workflow task executor's threads, which start with an empty context, so inside a workflow ADK still used wall-clock time and uuid.uuid4() for session, event, invocation, and function-call ids. Only debug mode, which runs activations inline, saw the deterministic values.

The plugin now rebinds each google.adk.platform ContextVar (time, uuid, and the random seam added in ADK 2.8.0) to one whose default is the Temporal provider, so it applies in every context on Worker and Replayer, sandboxed or not. Installation is idempotent; outside a workflow the providers fall back to the standard library.

Breaking: the google-adk extra now requires google-adk>=2.8.0, the first release with the _random seam.

Tested with a real Worker (sandboxed and unsandboxed) plus replay, and a fresh-thread check that fails on main. Lock and doc notes are in the first comment.

Same approach as _install_provider in #1675, which can rebase onto this.

[DABH]

Notes for review:

• uv.lock: besides google-adk 2.4.0 -> 2.8.0, the diff carries google-genai 2.11.0 -> 2.20.0 (ADK 2.8.0 requires google-genai>=2.19), a new aiohttp edge, and marker/exclude-newer re-serialization from a newer uv. uv lock --check passes.
• Behaviour documented in the CHANGELOG and README: installation warns if it replaces an ADK provider set earlier in the calling context (overrides must be made after the worker starts or from workflow code), and, like workflow.uuid4(), ADK id generation and get_random() in query handlers and update validators now raise ReadOnlyContextError instead of returning a random value.
• The README's "Support for custom span exporters" bullet is removed as a stale-doc cleanup, unrelated to the fix.
• Validation: uv run pytest tests/contrib/google_adk_agents -> 58 passed, 5 skipped (tests needing GOOGLE_API_KEY and the CI-skipped MCP tests); uv run poe lint (ruff, pyright, mypy, basedpyright, pydocstyle) passes.