Add PropertyDescriberInterface extension point for SchemaGenerator

[peter-si]

Summary

Adds a PropertyDescriberInterface extension point to SchemaGenerator so callers can teach it how to render specific class types (DateTime, Uuid, domain value objects) as targeted JSON Schema fragments rather than the generic {type: "object"} fallback.

Describers are consulted, in order, for class-typed parameters before generic class inspection. The first non-null result wins; parameter-level metadata (description, default, nullable) is layered onto the described schema without overwriting fields the describer already set.

Ships two default describers:

• Mcp\Capability\Discovery\PropertyDescriber\DateTimePropertyDescriber — any \DateTimeInterface implementation → {type: "string", format: "date-time"}
• Mcp\Capability\Discovery\PropertyDescriber\UuidPropertyDescriber — Symfony\Component\Uid\Uuid and subclasses → {type: "string", format: "uuid"}

Why

If an MCP tool method takes e.g. \DateTimeInterface \$until, the generator currently falls back to {type: "object"}, which tells the LLM nothing about the expected shape and forces ad-hoc workarounds in every tool. The describer chain gives a clean extension point for the long tail of value-object types every project has, without needing to subclass or fork `SchemaGenerator`.

Downstream we extend the chain further with app-specific types (Money, PhoneNumber, ...), but the two shipped here are general enough that they feel like they belong upstream.

Backwards compatibility

• New constructor parameter defaults to an empty iterable.
• Behavior is unchanged for callers that don't pass any describers.
• No public API removed or modified.

Tests

• New `DateTimePropertyDescriberTest` and `UuidPropertyDescriberTest` cover per-describer happy paths and pass-through.
• New integration tests in `SchemaGeneratorTest` cover: fallback when no describer claims the type, describer override of generic inference, docblock-description layering, nullable + default handling, first-non-null-wins ordering, and that describers don't intercept unrelated class types.
• Full `unit` suite: 690 tests, 2215 assertions, all green.

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