Make userAgent a true multi-stage routine

[alganet]

Turn userAgent() into a routine that can participate in multiple phases of the request lifecycle instead of only post-processing responses.

Changes:

• move negotiated callback storage/helpers into AbstractCallbackMediator so mediator-based routines can share state across stages
• make UserAgent implement ProxyableBy as well as ProxyableThrough
• allow userAgent callbacks to block before the route runs by returning false, a ResponseInterface, or a forwarded route
• preserve response transformation behavior through userAgent callbacks after the route runs
• avoid double-running zero-argument userAgent callbacks by preparing the through-stage result during by()
• preserve through-only behavior for callbacks that require a response/input parameter and therefore cannot run safely before the route

Tests:

• add router coverage for pre-route blocking via userAgent
• add coverage for a single userAgent routine participating in both by and through stages
• add coverage for transformer-style userAgent callbacks that remain post-route only
• extend UserAgent routine tests for deny behavior and prepared through results

Verification:

• composer qa

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.30%. Comparing base (89994e6) to head (8f3768c).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #163      +/-   ##
============================================
+ Coverage     96.10%   96.30%   +0.19%     
- Complexity      418      433      +15     
============================================
  Files            27       27              
  Lines          1027     1055      +28     
============================================
+ Hits            987     1016      +29     
+ Misses           40       39       -1     

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