Extract FileExtension routine from Accept content negotiation

[alganet]

Add a dedicated FileExtension routine that handles URL extension mapping (.json, .html, etc.) independently from Accept header negotiation. Only declared extensions are stripped during route matching, so dots in paths like /users/john.doe no longer get mangled.

Multiple FileExtension routines can cascade for compound extensions like .json.en — each peels its extension from right to left.

Decouple AbstractAccept from IgnorableFileExtension entirely, making it pure header-based content negotiation. Promote IgnorableFileExtension from marker interface to a real interface with getExtensions() so route matching knows exactly which extensions to strip.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.74468% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.67%. Comparing base (089f417) to head (8cf6a7b).

Files with missing lines	Patch %	Lines
src/Routines/FileExtension.php	92.59%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #175      +/-   ##
============================================
- Coverage     96.68%   96.67%   -0.02%     
- Complexity      418      430      +12     
============================================
  Files            30       31       +1     
  Lines          1056     1082      +26     
============================================
+ Hits           1021     1046      +25     
- Misses           35       36       +1     

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[Copilot]

Pull request overview

This PR separates URL file-extension handling from Accept header content negotiation by introducing a dedicated FileExtension routine and updating route matching so only declared extensions are stripped (preserving dots in normal path segments like john.doe).

Changes:

• Add src/Routines/FileExtension to map declared URL extensions (e.g. .json, .html) to response transforms, with support for cascading compound extensions (e.g. .json.en).
• Remove URL-extension behavior from AbstractAccept, making it purely header-based negotiation.
• Update route matching + tests/docs/examples to use fileExtension() and ensure only declared extensions are stripped.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/Routines/FileExtension.php	New routine for extension-based response transforms and extension “peeling”.
src/Routes/AbstractRoute.php	Updates matching to strip only declared extensions and stores remaining suffix for routines.
src/Routines/AbstractAccept.php	Removes URL-extension coupling; keeps negotiation header-only.
src/Routines/IgnorableFileExtension.php	Promotes marker interface to a real contract via getExtensions().
tests/Routes/AbstractRouteTest.php	Adjusts extension-stripping assertions to match new behavior.
tests/RouterTest.php	Renames/updates URL extension tests; adds cascade + leniency + exception/global-accept tests.
docs/README.md	Adds fileExtension() documentation section.
example/full.php	Adds a fileExtension() usage example for JSON/HTML.

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