Refine response metadata defaults for negotiated responses

[alganet]

Introduce a small request-level response metadata API so routines can communicate through replace/append/default header semantics instead of overwriting one shared header bag.

Changes:

• add Request helpers for explicit, appended, and default response headers
• apply appended headers by merging with handler-provided response headers and apply default headers only when the response does not already define them
• clear stale routine metadata between candidate route matches
• update Accept* routines to use the new metadata API
• stop emitting automatic Expires and Cache-Control headers for negotiated responses
• keep negotiated representation headers while making Content-Location a default that a handler may override
• preserve explicit response headers such as Cache-Control returned by the route handler

Tests:

• update negotiation metadata expectations to no longer require automatic cache

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.46%. Comparing base (7a9d34b) to head (a2fc582).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #164      +/-   ##
============================================
+ Coverage     96.30%   96.46%   +0.16%     
- Complexity      433      441       +8     
============================================
  Files            27       27              
  Lines          1055     1076      +21     
============================================
+ Hits           1016     1038      +22     
+ Misses           39       38       -1     

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