fix(ci): enable C++ server unit tests in CI

[cheese-cakee]

Context

test_server_unit is defined in server/CMakeLists.txt:874-902 and registered with ctest (line 901), but is never built or run in CI. The commented-out ctest block at .github/workflows/ci.yml:63-67 claims:

Server unit tests require libcurl-dev; skipped when CURL is absent.

This is incorrect. All curl usage in http_server.cpp is behind #ifdef DFLASH_HAS_CURL (lines 14, 60-254, 809, 954, 2187-2236). CMakeLists.txt conditionally defines DFLASH_HAS_CURL only when CURL_FOUND (lines 888-893). The test binary compiles and runs fine without curl — the proxy passthrough feature is simply disabled.

Bug

204 CPU-only tests covering critical server components are untested in CI:

Category	Tests	Critical path?
SSE emitter	19	Yes — every streaming response
Stop sequences	10	Yes — every request with stop tokens
Sampler	18	Yes — every non-greedy request
Prefix cache	5	Yes — prefix caching path
Jinja chat template	7	Yes — every chat request
Tool parser	33	Agent/tool-use flow
Reasoning parser	7	Reasoning mode
PFlash config	18	PFlash feature
Disk prefix cache	18	Disk-cache feature
Backend IPC	5	Remote-drafter IPC
+ 8 more sections	64	Various

Fix

1. Add test_server_unit to the cmake build targets (line 60)
2. Uncomment the ctest step (lines 63-67)
3. Remove the incorrect comment about libcurl-dev

Changes

• .github/workflows/ci.yml — added test_server_unit to build targets, uncommented ctest step

Verification

• All curl code in http_server.cpp is #ifdef DFLASH_HAS_CURL guarded (5 occurrences)
• CMakeLists.txt conditionally defines and links curl only when CURL_FOUND
• CI already installs CUDA toolkit via Jimver/cuda-toolkit@v0.2.35
• Test binary tests pure CPU logic: parsers, emitters, sampler, templates, cache hashing

Notes

• The test binary links against dflash_common which includes ggml-cuda, but CI has CUDA installed so this is fine
• No GPU or model files needed — tests exercise parser/emitter/sampler logic only

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}