refactor: extract router helpers into utils.py (closes #582)

[ScarsAndSource]

Description

Identified and extracted two repeated code patterns from the analysis routers into a new backend/app/routers/utils.py module:

Duplicate 1 — 13-line issue-count + payload block existed identically in debugging.py and analyze.py/_stream_analysis:

• Both files independently counted errors/warnings/infos by severity
• Both built the same summary string
• Both assembled the same response dict

Duplicate 2 — detect_language(req.code, req.language) repeated in every analysis router (debugging.py, explanation.py, suggestions.py)

Files Changed

Created:

• backend/app/routers/utils.py — new utilities module with two helpers:
  • build_debugging_payload(issues, code=None) — centralises severity counting, summary formatting, and payload assembly
  • resolve_language(req) — wraps detect_language(req.code, req.language) used across all routers
• backend/tests/test_router_utils.py — 10 unit tests covering all branches of both helpers

Replaced:

• backend/app/routers/debugging.py — removed 10 inline lines; now calls resolve_language() and build_debugging_payload()
• backend/app/routers/analyze.py — removed 13-line duplicate block inside _stream_analysis; now calls build_debugging_payload()
• backend/app/routers/explanation.py — swapped detect_language(req.code, req.language) → resolve_language(req)
• backend/app/routers/suggestions.py — swapped detect_language(req.code, req.language) → resolve_language(req)

No behaviour changes: all existing tests pass.

Related Issue

Fixes #582

Type of change

• Bug fix
• New feature / enhancement
• Documentation update
• Test addition
• Refactor

Checklist

• I have read CONTRIBUTING.md
• My branch is up to date with main
• I have run pytest -v and all tests pass
• I have not introduced duplicate issues or features
• My PR title follows the format: feat/fix/docs/test: short description
• I have added tests for new features (Level 2 and 3 issues)
• No hardcoded secrets or API keys in my code
• This PR is linked to a GSSoC 2026 issue

Screenshots (if frontend change)

Test evidence

pytest -v
# paste output