test(bdr): unit-test the Big Dumb Reply cache accessors #391

Background

The Big Dumb Reply cache (`include/cwist/sys/app/big_dumb_reply.h`) auto-caches high-latency endpoint responses, but its public accessors have no wired unit test and sit in `scripts/ci/api-untested-baseline.txt`:

• `cwist_bdr_put_fixed` — cache a fixed static response up front
• `cwist_bdr_get` — look up a cached response by method + path
• `cwist_bdr_get_pinned` / `cwist_bdr_unpin` — epoch-pinned lookup
• `cwist_bdr_set_limits` — cap cache bytes / entry age / revalidate hits

What to do

1. Create `tests/test_bdr_cache.c` using `cwist_bdr_create()` (no app/server needed — the cache is standalone).
2. Suggested cases:
   • `put_fixed` then `get` returns the same bytes and length; `get` on an unregistered path returns NULL.
   • `get_pinned` returns the blob plus a pin; the blob stays readable until `cwist_bdr_unpin`.
   • `set_limits` with small `max_bytes` evicts (or refuses) an oversized `put_fixed` — assert the documented behavior from the header comments.
   • Cache key is per-method: the same path registered for GET is not returned for POST.
3. Add a Makefile target (build + run, mirroring `test_app_named_routes_and_error_handler`) and add it to the test list.
4. Remove the covered names from `scripts/ci/api-untested-baseline.txt`.

Acceptance criteria

• `make test_bdr_cache` (or your target name) passes.
• `python3 scripts/ci/check_api_test_coverage.py` exits 0 with a higher tested count.

Pointers

• Header contract: `include/cwist/sys/app/big_dumb_reply.h` (doc comments state the exact lookup/pin/limit semantics — assert those, not assumptions)
• Test template: `tests/test_app_named_routes_and_error_handler.c` (test(app): smoke tests for named routes, healthz, and set_error_handler (#342) #388)