Validate function type annotations at runtime and add signature mismatch tests

[nbeerbower]

Motivation

• Ensure that when a value is annotated as a function type the runtime verifies the actual function matches the annotated signature (params/return/async/rest).
• Reuse existing Type structures and types_compatible logic for comparing function parameter and return types.
• Provide a clear runtime error that reports both the expected and actual function signatures to aid debugging.
• Add interpreter tests that exercise mismatched function signatures so the behavior is specified.

Description

• Added a helper type_matches_function_signature(Type *expected_type, Function *func, const char **error_msg) that constructs a temporary Type for the actual function and validates async flag, parameter count/types, rest param, and return type using types_compatible and type_to_string for formatting.
• Updated convert_to_type so when the target_type is TYPE_FUNCTION and the value is VAL_FUNCTION it calls the helper and emits a runtime error with both expected and actual signatures if validation fails.
• Reused existing type machinery (Type, types_compatible, type_to_string) rather than introducing new signature representations.
• Added interpreter tests tests/functions/function_signature_param_error.hml and tests/functions/function_signature_return_error.hml with corresponding .expected files asserting the runtime error messages.

Testing

• Added two interpreter tests: tests/functions/function_signature_param_error.hml and tests/functions/function_signature_return_error.hml with expected outputs showing the mismatch messages.
• No automated test runner was executed as part of this change (tests were added but not run).
• The new runtime error message format is: Function signature mismatch (expected <expected_sig>, got <actual_sig>).
• Manual build/CI execution is recommended to run the full test suite and verify no regressions.

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Codex Task