Cast variadic tool parameters element-by-element

[Faneraiy14]

Summary

SchemaGenerator::buildVariadicParameterSchema() already advertises a variadic parameter (e.g. int ...$scores) as a JSON "array" schema, so a spec-conformant client sends an array for it. But ReferenceHandler::prepareArguments() never special-cased ReflectionParameter::isVariadic() — it fell through to the regular single-value branch and passed the whole array to castArgumentType(), which tried to cast it as one scalar and failed (e.g. "Cannot cast value to integer" for int ...$scores given [1, 2, 3]).

Any tool handler declaring a variadic parameter was therefore unreachable through the advertised schema — confirmed by direct reproduction against ReferenceHandler::handle() before this change:

EXCEPTION: Mcp\Exception\RegistryException: Cannot cast value to integer. Expected integer representation.

This resolves the // TODO: Handle variadic parameters. left in prepareArguments().

Fix

Casts each array element individually via the existing castArgumentType() (unchanged — it already inspects the variadic parameter's element type, not array, per PHP reflection semantics) and appends to the result in order. Variadic is always the last parameter, so this preserves correct positional ordering. A non-array value for a variadic argument now fails clearly with an invalid-params error instead of the confusing cast message.

Test plan

• New unit tests: array argument cast element-by-element, omitted argument treated as zero elements, non-array argument rejected with a clear error
• vendor/bin/phpunit --testsuite=unit — 1516 tests, 3923 assertions, all passing
• vendor/bin/phpstan analyse (full repo, level 6 per phpstan.dist.neon) — no errors