Add provider-priority ordering and runtime fallback

[georgestephanis]

Tying in to https://core.trac.wordpress.org/ticket/65358 -- felt like it'd be better to offer the PR upstream instead of in the core repo?

Anyway, this has been largely aided by Claude and Copilot, based on an issue and problem I was facing (Detailed upstream, but --) frustration that I can't manage prioritization of use between multiple providers.

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 95.23810% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.17%. Comparing base (99a65e8) to head (915b0c6).

Files with missing lines	Patch %	Lines
src/Builders/PromptBuilder.php	94.79%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #239      +/-   ##
============================================
+ Coverage     88.12%   88.17%   +0.04%     
- Complexity     1213     1235      +22     
============================================
  Files            60       60              
  Lines          3934     3990      +56     
============================================
+ Hits           3467     3518      +51     
- Misses          467      472       +5     

Flag	Coverage Δ
unit	88.17% <95.23%> (+0.04%)	⬆️

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