Return false from isSupported() when the output modality has no matching capability

[jigneshbhavani]

Fixes #270

isSupported() is declared : bool, but the RuntimeException from inferCapabilityFromOutputModalities() propagates straight out of it when the configured output modality has no matching capability. ModalityEnum::document() is the reachable case, since DOCUMENT is a first class modality but CapabilityEnum has nothing to map it to.

This catches that one call and returns false. Asking whether an unsupported modality is supported has a correct answer, and it is no.

Two things kept deliberately narrow:

• generateResult() is untouched and still throws for the same modality. That is the right behaviour there, and testGenerateResultThrowsExceptionForUnsupportedOutputModality() asserts it.
• Only the inference call sits inside the try, so a RuntimeException raised anywhere else in isSupported() still propagates.

I also considered changing inferCapabilityFromOutputModalities() to return ?CapabilityEnum and letting each caller decide, which reads a little cleaner, but it changes a shared private method used by the generation path as well. Happy to redo it that way if you would rather.

Testing

testIsSupportedReturnsFalseForUnsupportedOutputModality() fails on trunk with:

WordPress\AiClient\Common\Exception\RuntimeException: Output modality "document" is not yet supported.

src/Builders/PromptBuilder.php:556
src/Builders/PromptBuilder.php:616
tests/unit/Builders/PromptBuilderTest.php:3877

and passes with the patch. Full unit suite green at 1165 tests, 4261 assertions. phpcs and phpstan both clean.

I did not add an @since line to the docblock since I do not know which version this would land in. Say the word and I will add it.

I checked the open PRs that touch nearby code and there is no overlap. #222 adds isSupportedForSoundGeneration(), which passes an explicit capability and never reaches the inference branch, and #254 changes ModelResolver::isSupported(), which is a different method.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.50%. Comparing base (a31b0ec) to head (f9f2bec).

Additional details and impacted files

@@            Coverage Diff            @@
##              trunk     #271   +/-   ##
=========================================
  Coverage     86.49%   86.50%           
- Complexity     1327     1328    +1     
=========================================
  Files            68       68           
  Lines          4295     4297    +2     
=========================================
+ Hits           3715     3717    +2     
  Misses          580      580           

Flag	Coverage Δ
unit	86.50% <100.00%> (+<0.01%)	⬆️

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