Make integration tests adaptive to any indexed repository

Tests were hardcoded for codealive-app (.cs extensions, CodeAlive.Domain
paths, C#-specific grep patterns) and failed against other repos.

Changes:
- Add _discover_target_content() that probes the target repo to find
  actual extensions and path prefixes via a semantic search sample
- Replace all hardcoded .cs/CodeAlive.Domain with discovered values
- Fix assertion bugs: use r.isError for validation checks (max_results
  bounds, invalid profile), not just string matching
- Fix empty data_sources=[] test: backend returns 404 with actionable
  hint, not a fallback — test now verifies the hint content

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: rodion-m