fix: return tool errors for invalid arguments

[0xWeakSheep]

Closes #840.

Tool argument deserialization failures now return a tools/call result with isError: true instead of a JSON-RPC -32602 error.

This lets clients pass the validation message back to the model so it can retry with corrected arguments.

Only parameter deserialization errors are converted. Other protocol errors, such as unknown tools, still return JSON-RPC errors.

Checks run:

cargo +nightly fmt --all --check
git diff --check
cargo test -p rmcp handler::server::router::tool::tests::test_argument_deserialization_error_returns_tool_error_result
cargo test -p rmcp handler::server::router::tool::tests::test_call_disabled_tool_returns_error