Add path validation to functions proxy convention

[todo-mdx]

SECURITY: The functions proxy in src/conventions/functions/proxies.ts does not use validateProxyPath() like the main proxy convention, allowing potential SSRF attacks.

Location: src/conventions/functions/proxies.ts lines 70-73, 124

TDD Steps:

• RED: Write test attempting path traversal through functions proxy
• GREEN: Apply validateProxyPath() from src/helpers/path.ts to functions proxy
• REFACTOR: Ensure consistent path validation across all proxy patterns