refactor(mcp): extract tool test modules

[xxx]

Summary

• extract the shared MCP tool test platform into tools/testutil.rs
• extract the tools unit tests into tools/tests.rs
• reduce tools.rs from 2,267 lines to 503 without changing production behavior

Validation

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo test --workspace --locked
• verified the extracted modules preserve the original token stream apart from formatting

Review

Reviewed against origin/master for repository standards and the behavior-preserving refactor request. No findings.