Use unique temporary files for code runs

[flaviens]

Summary

• Create a per-run temporary directory and source file instead of reusing one filename per language.
• Execute the configured interpreter with execFile and an argument array.
• Remove the temporary directory after each tool invocation completes.

Rationale

The previous implementation reused a shared temporary filename for each language. Overlapping run-code calls could interfere with each other, and temporary files were left behind after execution. This change isolates each invocation and cleans up its temporary files.

Validation

• npm ci
• npm run build
• MCP in-memory smoke test with two concurrent run-code JavaScript calls returning independent alpha / beta outputs and leaving 0 new mcp-code-runner-* temp directories.

Note: npm ci reports existing dependency audit findings (2 moderate, 3 high); this PR does not change dependencies.

[flaviens]

Superseded by #25 with a cleaner branch name and commit message.