[Feature] /scan-url: add tests for URL validation and reachability checks

[lakshay122007]

Summary

Add unit tests for the /scan-url URL validation and reachability check logic added in #10.

Motivation

The validation logic in scan_url currently has no test coverage. Without tests, regressions in URL format checking, reachability detection, or timeout handling could go unnoticed.

Proposed solution

Add tests in the backend test suite that mock httpx to cover the following cases without making real network requests:

• Invalid URL format returns 422
• Non-existent repo (404 from GitHub HEAD request) returns 422
• Network timeout returns 422 with "Could not reach GitHub" message
• Valid public repo passes reachability check and proceeds normally

ML tier (if applicable)

• Tier 1 — Triage (severity ranking, deduplication, false positive classification)
• Tier 2 — Predictive (fix success prediction, exploit scoring, pattern clustering)
• Tier 3 — Autonomous (LLM patch generation, self-healing pipeline)
• Not ML-related

Acceptance criteria

• Tests cover invalid URL format, 404, timeout, and 200 cases
• httpx is mocked — no real network calls made during tests
• Tests pass in CI across Python 3.10, 3.11, 3.12

Additional context

Follow-up to PR #10.

@ionfwsrijan Please assign this issue to me. thanks!

[ionfwsrijan]

@lakshay122007 Done!

[lakshay122007]

btw - i am not able to add labels for it for Ssoc'26 and is there any discord channel created where we can discuss about problems?

[ionfwsrijan]

@lakshay122007 You may dm me on dc: ionfwsrijan_ . Just in-case if you've any queries.

[ionfwsrijan]

@lakshay122007 Talking about the labels thing. The PAs are only supposed to do that not the contributors. You aren't supposed to touch those things.