refactor: Add name validation to file accessor

[gavin-aguiar]

Description

This PR improves the robustness of  FileAccessorUnix  in the shared memory data transfer path by adding stricter validation of memory map names and ensuring resolved file paths stay within the configured shared memory directories.

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):