fix(http/unstable): improve Cache Control parsing and types

[tomas-zijdemans]

Breaking type changes:

• Removed CacheControlBase; RequestCacheControl and ResponseCacheControl are now flat interfaces
• CacheControl is now a merged mapped type instead of a union
• staleIfError added to RequestCacheControl (per RFC 5861)

Bug fixes:

• Handle backslash escapes inside quoted strings in splitDirectives
• Normalize no-cache="" and private="" to true instead of []

Perf:

• Hoist digit regex to module scope
• Replace Set-based duplicate tracking with direct property checks

Tests: added coverage for duplicate directives (all branches), escaped quotes, empty field-name normalization, and round-trip stability.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.41%. Comparing base (1cd63ca) to head (197ce50).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7087   +/-   ##
=======================================
  Coverage   94.41%   94.41%           
=======================================
  Files         630      630           
  Lines       50490    50516   +26     
  Branches     8949     8972   +23     
=======================================
+ Hits        47669    47696   +27     
  Misses       2249     2249           
+ Partials      572      571    -1     

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