Add baggage propagation tests for Flask and WSGI instrumentations

[jojinkb]

Description

The Flask instrumentation and the WSGI OpenTelemetryMiddleware extract W3C baggage from
incoming request headers (through the default composite propagator used by
_start_internal_or_server_span) and make it active for the duration of the request. This
has worked for a long time, but there was no unit test covering it — neither package
referenced baggage anywhere in its tests.

This PR adds the tests requested in the issue:

• WSGI (test_wsgi_middleware.py):
  • test_baggage_extracted_and_active_in_wrapped_app: baggage entries from the incoming
    baggage header (including a percent-encoded value) are available via
    baggage.get_all() inside the wrapped WSGI application.
  • test_baggage_detached_after_response_is_consumed: the extracted context is detached
    after the response iterable is consumed, so baggage does not leak past the request.
• Flask (base_test.py, test_programmatic.py):
  • test_baggage_extracted_and_active_in_handler: baggage from the incoming request
    header is available via baggage.get_all() inside the route handler (new /baggage
    test route), and the context is detached once the request has ended.

Tests-only change, no functional changes — no changelog fragment (Skip Changelog).
The # pylint: disable=too-many-public-methods on TestWsgiApplication was needed
because the class was already at the 20-method limit; the same disable is already used on
TestWsgiAttributes in the same file.

Fixes #268

Type of change

• Tests only (no functional change)

How Has This Been Tested?

• tox -e py312-test-instrumentation-wsgi equivalent: full WSGI suite — 54 passed
  (52 baseline + 2 new).
• tox -e py312-test-instrumentation-flask-3 equivalent: full Flask suite — 61 passed,
  1 skipped (60 baseline + 1 new); also re-run against the oldest tested versions
  (Flask==2.1.3, Werkzeug==2.3.8, the flask-0 pins) — 61 passed, 1 skipped.
• Negative check: with OTEL_PROPAGATORS=tracecontext (baggage propagator removed from
  the composite), both new extraction tests fail — confirming they guard the behavior.
• Lint: ruff check / ruff format --check (v0.14.1) clean on changed files;
  pylint --rcfile .pylintrc (v4.0.5) 10.00/10 on both packages.

Does This PR Require a Core Repo Change?

• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated (not needed — tests only, requesting Skip Changelog label)
• Unit tests have been added
• Documentation has been updated (not applicable)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: jojinkb / name: Jojin (1cbcdbb)