style(clippy): fix test file lints (zombie_processes, empty_line_after_doc_comment, useless_format, unused_variable)

[github-actions]

What clippy found

Running cargo clippy --all-targets --all-features --workspace -- -D warnings reported four findings in the test files:

error: empty line after doc comment
  --> tests/mcp_http_tests.rs:9:1

error: spawned process is not `wait()`ed on in all code paths
  --> tests/mcp_http_tests.rs:56:21
  note: this if expression has a `wait()` call, but it is missing an else block
  --> tests/mcp_http_tests.rs:84:9

error: unused variable: `output_yaml`
  --> tests/compiler_tests.rs:46:9

error: useless use of `format!`
  --> tests/compiler_tests.rs:3733:24
  help: consider using `.to_string()`: `"\"source\":\"".to_string()`

How it was fixed

Lint	File	Fix
clippy::empty_line_after_doc_comment	tests/mcp_http_tests.rs	Changed module-level /// to // — the block describes the test module, not a specific item, so // is correct
clippy::zombie_processes	tests/mcp_http_tests.rs	Added child.wait().ok() after child.kill().ok() before the panic! so all code paths reap the child process
clippy::unused_variables	tests/compiler_tests.rs	Prefixed output_yaml with _ to signal intentional non-use
clippy::useless_format	tests/compiler_tests.rs	Replaced &format!("\"source\":\"") with the string literal "\"source\":\"" directly

Verification

• cargo build --all-targets: ✅ succeeded
• cargo test --test mcp_http_tests: ✅ 8 passed, 0 failed
• cargo test --test compiler_tests: ✅ 116 passed, 0 failed
• cargo clippy --all-targets --all-features --workspace -- -D warnings: ✅ no new warnings in the changed files

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