fix: ignore proxy_protocol listen sockets during api discovery (#978)

[devaniketh]

Proposed changes

When NGINX is configured with listen ... proxy_protocol;, connecting to that socket requires an immediate PROXY protocol header. The agent's config parser previously created plain HTTP endpoint URLs for these sockets, causing NGINX connection resets (broken header: "GET /api HTTP/1.1" while reading PROXY protocol) and agent scraping failures (EOF).

This PR:

• Detects the proxy_protocol parameter on listen directives via hasProxyProtocolArgument and isProxyProtocolListenDirective.
• Excludes proxy_protocol listen sockets when generating plain HTTP endpoints for /stub_status and /api.
• Allows mixed configurations (e.g. proxy_protocol on port 8888 and standard listener on port 8080 or unix socket) to discover the valid, reachable endpoint.
• Adds table-driven unit tests for both mixed listeners and proxy_protocol-only listeners in nginx_config_parser_test.go.

Closes #978

Checklist

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes

[github-actions]

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[devaniketh]

I have hereby read the F5 CLA and agree to its terms

[devaniketh]

recheck

[devaniketh]

@aknot242 hey kindly check the PR