Support opt-in strict content negotiation

[joao-jtjsoftware]

Summary

• add opt-in strict HTTP content negotiation through server.api_rules.strict_content_negotiation
• return 406 Not Acceptable for unsupported Accept media types in strict mode
• preserve the existing default representation fallback when strict mode is disabled
• support / and type wildcards such as image/*
• apply the setting consistently to Flask, Starlette and Django
• document and validate the new configuration option

Behaviour

The option defaults to false, so existing deployments retain their current behaviour. Invalid ?f= values continue to return 400 InvalidParameterValue.

Tests

• pytest tests/other/test_config.py tests/api/test_api.py::test_apirequest tests/api/test_api.py::test_apirules_active tests/api/test_api.py::test_apirules_inactive -q
• flake8 --jobs 1 pygeoapi tests/api/test_api.py
• git diff --check

All targeted checks pass.

Related to #2374. This is a draft pending agreement on the opt-in approach discussed in the issue.