feat(genai): production-grade model-agnostic FallbackPolicy middleware

[giterinhub]

Summary & Context

This PR introduces and enhances the Model-Agnostic Fallback Middleware (FallbackPolicy) for the google-genai Python SDK.

The policy enables automatic failovers across dynamic model sequences and Vertex AI regions when encountering retryable API errors (e.g. 429 Rate Limit, 503 Service Unavailable, 504 Gateway Timeout).

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Key Improvements & Enhancements

1. Universal Exception Code Extraction:

   • _extract_status_code dynamically inspects APIError.code, exc.status_code, and exc.response.status_code, enabling seamless compatibility across google-genai, httpx, requests, and google.api_core.

2. Telemetry & Audit Callback (on_fallback):

   • Accepts an optional on_fallback(exc, attempt_idx, next_payload) hook allowing downstream applications to record metrics (Prometheus, OpenTelemetry, Datadog) or custom failover audit logs.

3. Configurable Backoff (backoff_factor):

   • Supports exponential backoff delays (backoff_factor * 2^(attempt-1)) for both synchronous (time.sleep) and asynchronous (asyncio.sleep) execution paths.

4. Payload Immutability Guarantee:

   • Each attempt uses copy.deepcopy(kwargs) to ensure nested objects (contents, config) remain completely unmutated across retry iterations.

5. Multi-Dimensional Fallback Matrix:

   • Sequential matrix evaluation when both fallback_models and fallback_locations are specified, evaluated up to max_retries.

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Verification & Testing

• Ruff Linting: python3 -m ruff check google/genai/_fallback.py google/genai/tests/test_fallback.py google/genai/tests/test_fallback_unittest.py -> Passed (0 errors)
• Ruff Formatting: python3 -m ruff format --check google/genai/_fallback.py google/genai/tests/test_fallback.py google/genai/tests/test_fallback_unittest.py -> Passed (0 unformatted files)
• Pytest Suite: pytest google/genai/tests/test_fallback.py -v -> 13 / 13 Passed
• Unittest Suite: python3 google/genai/tests/test_fallback_unittest.py -> 12 / 12 Passed
• Concurrency Stress Test: 100 concurrent async tasks executed through execute_async via asyncio.gather().

[giterinhub]

Note on mypy 3.11 static checking:
mypy flags 9 pre-existing type errors in google/genai/types.py and google/genai/tunings.py

caused by static AST evaluation of optional try: 
from IPython import display
except
ImportError: IPython_display = None branches and untyped IPython calls when strict checking individual files.

To keep this PR strictly targeted to FallbackPolicy, those 9 pre-existing IPython type fixes have been isolated into a separate PR: #2773.