Name the unmet option, not just the capability, in resolution failures

[sarthak-19]

Summary

ModelResolver::resolve() built its "no models found" failure message from the primary required capability only. When resolution actually failed because a required option (e.g. webSearch, inputModalities) eliminated every candidate, the message still blamed the capability — which those models did in fact support. That makes the message actively misleading: checking whether the capability is supported confirms that it is.

ModelRequirements::getUnmetRequirements() already exists specifically so calling code can report why a model is unsuitable, but ModelResolver's failure path never called it.

Change

On an empty candidate map, before throwing:

• Re-run the lookup using only the required capabilities (dropping required options).
• If that also returns nothing, the capability itself is unsupported and the message is unchanged.
• If it returns models, intersect getUnmetRequirements()['options'] across them and append a clause naming the option(s) that every one of them fails to support.

The extra lookup only happens on a path that's already about to throw, so it costs nothing in the normal case.

Before:

No models found for provider "acme" that support text_generation.

After (when webSearch is what actually eliminated every candidate):

No models found for provider "acme" that support text_generation. The following requested option is not supported by any of those models: webSearch.

Test plan

• composer test:unit — 1205 tests pass, including new ModelResolverTest cases: option named (unscoped and provider-scoped), several options named, and capability genuinely unsupported (no option named).
• PromptBuilderTest::testGenerateResultWithProviderNoModelsThrowsException updated: the registry mock's expectation widens from once() to exactly(2) since fromPromptData() always adds inputModalities as a required option, so the diagnostic re-lookup fires; the thrown message is unchanged since that second lookup also finds nothing.
• composer lint (phpcs + phpstan) — clean.

[codecov]

Codecov Report

❌ Patch coverage is 97.05882% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.66%. Comparing base (77a5995) to head (b726973).

Files with missing lines	Patch %	Lines
src/Providers/ModelResolver.php	97.05%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #298      +/-   ##
============================================
+ Coverage     86.58%   86.66%   +0.07%     
- Complexity     1383     1394      +11     
============================================
  Files            69       69              
  Lines          4449     4483      +34     
============================================
+ Hits           3852     3885      +33     
- Misses          597      598       +1     

Flag	Coverage Δ
unit	86.66% <97.05%> (+0.07%)	⬆️

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