Added a Behave feature file and complete step definitions that validate the
context tier hydrator can index projects with 10,000+ files without timing
out. Covers both the git ls-files (30-second timeout) path and the os.walk
fallback path, as well as binary file filtering and oversized file skipping.
Satisfies the v3.4.0 milestone acceptance criterion: "Projects with 10,000+
files index without timeout."
ISSUES CLOSED: #8726