[azure-core] Keep CaseInsensitiveSet consistent for in-place set operations

[Sebastian Turner (auraseb)]

Description

CaseInsensitiveSet (used for HttpLoggingPolicy.allowed_header_names and the allowed_query_params sets of HttpLoggingPolicy and DistributedTracingPolicy) subclasses set and answers in from a separate lowercase cache. The inherited |=, -=, &=, ^=, difference_update, intersection_update and symmetric_difference_update modify the base set without updating that cache, so policy.allowed_header_names |= {"x-custom-header"} leaves the header redacted and policy.allowed_header_names -= {"x-ms-request-id"} leaves it logged. Both worked while these attributes were plain sets.

This change overrides those methods so they go through add and discard, which keep the cache in sync. Matching is case-insensitive, consistent with the existing discard and remove. The in-place operators return NotImplemented for non-set operands, matching the built-in set.

Tests added in tests/test_utils.py for each operation, and in tests/test_http_logging_policy.py for the user-facing logging behavior. A CHANGELOG entry is added under Bugs Fixed.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

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• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

Fixes #49607

[github-actions]

Thank you for your contribution Sebastian Turner (@auraseb)! We will review the pull request and get back to you soon.

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[Copilot]

🟡 Changes recommended

Self-subtraction raises during iteration and can leave allowlists only partially cleared.

1 open issue

• Self-difference update partially clears sets and raises RuntimeError · New

What changed in this PR

Fixes #49607 by keeping CaseInsensitiveSet membership checks consistent with in-place mutations used by logging and tracing allowlists.

Changes:

• Routes in-place operators and update methods through cache-aware mutations.
• Adds utility and logging regression tests.
• Documents the fix in the changelog.

File	Description
sdk/​core/​azure-core/​tests/​test_utils.py	Tests case-insensitive set mutations.
sdk/​core/​azure-core/​tests/​test_http_logging_policy.py	Tests header redaction after allowlist changes.
sdk/​core/​azure-core/​CHANGELOG.md	Documents the allowlist fix.
sdk/​core/​azure-core/​azure/​core/​utils/​_utils.py	Adds cache-aware mutation overrides.

_{Review effort: Balanced}

[Copilot]

🔵 Needs a closer look

Self-subtraction still raises an iteration error instead of clearing the set.

1 open issue

• Self-difference update partially clears sets and raises RuntimeError

_{Review effort: Balanced}