Propagate PolicyEngine request IDs to the simulation service

[anth-volk]

Fixes #3781

Summary

• standardize API v1 request correlation on X-PolicyEngine-Request-Id
• share request IDs safely between the ASGI shell and mounted Flask application
• forward the active request ID through one HTTPX request hook to every simulation gateway request
• add regression coverage for supplied/generated IDs, alias rejection, context isolation, both simulation entrypoints, and calls outside request contexts

Root cause

API v1 used X-Request-ID, while the simulation observability stack reads X-PolicyEngine-Request-Id. Flask and ASGI also had separate request-ID handling, and the shared simulation client did not forward request-scoped correlation metadata.

Impact

Synchronous API v1, Simulation Entrypoint, and Modal gateway requests can now share one observability-supported request ID. Job and batch IDs remain unchanged for asynchronous execution correlation. This PR does not change routing, URLs, authentication, secrets, workflows, or API schemas.

Tests

• 15 focused request-ID tests passed
• 91 affected migration logging, ASGI, and simulation-client tests passed (one unrelated heavyweight application-import test excluded locally)
• 190 simulation contract, economy, budget-window, and deployment tests passed
• migration quality guards passed; generated migration contracts were unchanged
• Ruff formatting and changed-code lint passed
• towncrier draft and Python compilation passed
• Cloud Run Docker image build passed

[codecov]

Codecov Report

❌ Patch coverage is 97.67442% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.84%. Comparing base (c435b46) to head (fe7f66e).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
policyengine_api/migration_logging.py	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3782      +/-   ##
==========================================
+ Coverage   80.71%   80.84%   +0.13%     
==========================================
  Files          74       75       +1     
  Lines        4422     4448      +26     
  Branches      822      825       +3     
==========================================
+ Hits         3569     3596      +27     
+ Misses        623      622       -1     
  Partials      230      230              

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