[#281] Add provider-agnostic deferred tool loading

[superdav42]

Summary

• add an opt-in deferLoading flag to function declarations while preserving eager-loading defaults and serialized compatibility
• add native tool-search configuration and model requirements so unsupported models are rejected when deferred loading is requested
• add provider-scoped opaque message data so provider-native discovery items can be preserved in order without exposing provider wire formats in core

Scope

This PR defines the provider-agnostic contract only. OpenAI and Anthropic request serialization, response parsing, and model metadata updates remain provider-repository follow-ups. Client-executed tool discovery is intentionally excluded.

Implementation notes

• Existing FunctionDeclaration calls remain valid, and deferLoading: false is omitted from serialized output.
• PromptBuilder::usingToolSearch() explicitly requests native discovery.
• Any deferred declaration also infers a required toolSearch model option, preventing silent eager fallback.
• ProviderData records the originating provider ID with an opaque serializable payload; providers must validate ownership before replay.

Verification

• composer test:unit — 1,181 tests, 4,350 assertions passed
• composer lint — PHPCS and PHPStan passed

Resolves #281

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.05%. Comparing base (0a9a1b3) to head (ac965d2).

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #282      +/-   ##
============================================
+ Coverage     86.60%   87.05%   +0.44%     
- Complexity     1335     1356      +21     
============================================
  Files            68       69       +1     
  Lines          4314     4402      +88     
============================================
+ Hits           3736     3832      +96     
+ Misses          578      570       -8     

Flag	Coverage Δ
unit	87.05% <100.00%> (+0.44%)	⬆️

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