refactor: decompose func_metadata() into smaller focused helpers

[perhapzz]

Summary

Refactors the monolithic func_metadata() function in func_metadata.py into smaller, well-documented helper functions for improved readability, testability, and extensibility.

Motivation

Closes #1700

func_metadata() handled multiple responsibilities in a single ~150-line function: signature introspection, parameter model construction, return type analysis, type-based model dispatch, and schema generation. This made it difficult to understand, test individual behaviors, and extend with new type handling.

Changes

Extracted the following focused helper functions:

Function	Responsibility
_get_function_signature()	Signature introspection with error handling
_build_arg_model()	Parameter model construction (annotation, defaults, alias handling)
_resolve_return_annotation()	Return type validation and CallToolResult/Annotated unwrapping
_create_output_model()	Type-based dispatch to appropriate model creation strategy
_try_generate_strict_schema()	Schema generation with StrictJsonSchema error handling

Simplified _try_create_model_and_schema() to delegate to the new helpers.

The top-level func_metadata() is now a concise orchestrator (~20 lines) that calls these helpers in sequence.

Testing

• All 33 existing tests in test_func_metadata.py pass without modification
• No behavioral changes — this is a pure refactor (extract method)
• The public API (func_metadata(), FuncMetadata, ArgModelBase) is unchanged

Notes

• The file path has moved from src/mcp/server/fastmcp/ to src/mcp/server/mcpserver/ since the issue was filed — the refactor targets the current location.
• A potential follow-up could introduce a strategy/registry pattern for type handling in _create_output_model(), as suggested in the issue. Kept out of this PR to minimize scope.