feat(api): add X-Reflex-Request-ID header middleware

[huynna12]

Description

Adds an HTTP middleware that generates a UUID4 per request and attaches it as X-Reflex-Request-ID on all HTTP responses. The same ID is recorded on the OTel act span as tether.request_id for fleet-scale log tracing.

Closes #93

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Infrastructure / CI update

How Has This Been Tested?

• pytest tests/ passes locally
• tether doctor sanity check
• Other (please specify):

Description:

Added 5 unit tests in tests/test_request_id_header.py covering:

• Header present on /health and /act responses
• Header value is a valid UUID4
• Each request gets a unique ID
• Context variable holds the correct ID inside the route handler

Run with: pytest tests/test_request_id_header.py -v

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have kept the PR scoped to a single concern (one concern per PR)