fix: route workflow IDs and retry jitter through platform seams

[DABH]

Describe the bug

google.adk.platform provides injectable time and uuid providers so that embedders can customize how generated values are produced - for example to make agent runs deterministic and reproducible for testing, simulation, or auditing. Event IDs, invocation IDs, timestamps, and client function-call IDs already route through them, but three call sites on the workflow HITL/retry path still call the stdlib directly, so they bypass any installed provider:

1. events/request_input.py - RequestInput.interrupt_id defaults to uuid.uuid4(). This is the ID that recorded user FunctionResponses reference on resume; an ID minted outside the installed provider can't be reproduced, which breaks interrupt matching.
2. workflow/_tool_node.py - ToolContext.function_call_id is minted with uuid.uuid4() instead of platform_uuid.new_uuid() (which flows/llm_flows/functions.py already uses for the same purpose).
3. workflow/utils/_retry_utils.py - retry backoff jitter draws from the global random module. Jitter defaults to 1.0 for any RetryConfig, so every retried node computes an unreproducible delay.

Describe the fix

• Add a platform random seam - google.adk.platform._random with get_random() / set_random_provider() / reset_random_provider(), exported from google.adk.platform per the visibility convention - mirroring the existing time/uuid providers (ContextVar-based, default = stdlib random.Random()).
• Route the three call sites through the platform seams. Default behavior is unchanged (same uuid4/uniform semantics when no provider is installed).
• Migrate the two existing jitter tests from mock.patch('random.uniform', ...) to injecting a mock via set_random_provider, exercising the new seam with the exact same assertions.

This follows the injectable-provider approach also adopted in google/adk-java#1255.

Testing plan

New unit tests:

• tests/unittests/platform/test_random.py - default provider, custom provider, reset (mirrors test_uuid.py).
• tests/unittests/events/test_request_input.py - default interrupt_id is a uuid; minted via the platform id provider; explicit id preserved.
• tests/unittests/workflow/test_tool_node.py::test_tool_node_function_call_id_uses_platform_id_provider - the tool receives a provider-minted function_call_id.
• tests/unittests/workflow/utils/test_retry_utils.py::test_jitter_uses_platform_random_provider - a seeded provider makes the computed delay reproducible.

$ pytest tests/unittests/workflow/ tests/unittests/platform/ tests/unittests/events/
791 passed, 11 skipped, 10 xfailed in 6.95s

All pre-commit hooks (isort, pyink, addlicense, new-file-prefix, ADK compliance, codespell) pass.

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