fix: Make vendored google.protobuf a conditional fallback

[gavin-aguiar]

Description

Make the vendored copy a conditional fallback that only activates when the function app actually ships google.protobuf (the only scenario where the original descriptor-pool conflict can occur).

Detection

A single os.path.isdir check on:
/.python_packages/lib/site-packages/google/protobuf

If present → activate vendored fallback. If absent → do nothing; worker uses its own protobuf + upb.

Activation

When activation triggers:

1. Set PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=python before any vendored module is imported.
2. Pre-import the vendored modules and alias them into sys.modules under google.protobuf and google.protobuf.internal. Not google itself — that would shadow google.cloud.*, google.auth, google.api_core, etc. (the most common reason a customer ends up with protobuf in their tree in the first place).

The alias propagates to submodules automatically via the parent package's path, so from google.protobuf.timestamp_pb2 import Timestamp correctly resolves to the vendored copy without needing per-submodule aliasing.

Fixes #

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Pull Request Checklist

Host-Worker Contract

• Does this PR impact the host-worker contract (e.g., gRPC messages, shared interfaces)?
  • If yes, have the changes been applied to:
    • azure_functions_worker (Python <= 3.12)
    • proxy_worker (Python >= 3.13)
  • If no, please explain why:

Worker Execution Logic

• Does this PR affect worker execution logic (e.g., function invocation, bindings, lifecycle)?
  If yes, please answer the following:

Python Version Coverage

• Does this change apply to both Python <=3.12 and 3.13+?
• If yes, have the changes been made to:
  • azure_functions_worker (Python <= 3.12)
  • runtimes/v1 / runtimes/v2 (Python >= 3.13)
• If no, please explain why:

Programming Model Compatibility (for Python 3.13+)

• Does this change apply to both:
  • V1 programming model (runtimes/v1)?
  • V2 programming model (runtimes/v2)?
• Explanation (if limited to one model):