test(bdr): unit-test BDR cache accessors (closes #391)

[Bhumika-1432006]

What it does

Adds tests/test_bdr_cache.c covering the four cache accessors listed in scripts/ci/api-untested-baseline.txt:

• cwist_bdr_put_fixed + cwist_bdr_get — stores a fixed response; hit returns the same bytes and length; miss on an unregistered path returns NULL.
• Method-keyed lookup — the same path registered for GET is not returned for POST.
• cwist_bdr_get_pinned + cwist_bdr_unpin — pinned hit returns blob; blob stays readable while pin is held; cwist_bdr_unpin(NULL) does not crash; miss returns NULL with a NULL pin.
• cwist_bdr_set_limits — sets max_bytes smaller than a test entry; asserts no crash on oversized put and on zero-value arguments.

cwist_bdr_create() needs no server — the cache is standalone, per the header.

How verified

make test_bdr_cache

Compiles and exits 0.

python3 scripts/ci/check_api_test_coverage.py

Exits 0; four names removed from scripts/ci/api-untested-baseline.txt.

What this does NOT do

Does not test concurrent or multi-threaded put paths (covered by the existing test_bdr), disk-mode, or the revalidation hook.