fix(jit): propagate -DNDEBUG to host-side cflags (#3278)

## 📌 Description

`gen_jit_spec` adds `-DNDEBUG` only to `extra_cuda_cflags` (consumed by
`nvcc` for `.cu` files), not to `extra_cflags` (consumed by `g++` for
host-side `.cpp`). Several host-only translation units are part of
MoE/GEMM JIT specs — most notably
`csrc/nv_internal/cpp/common/logger.cpp` — and they end up compiled
without `NDEBUG` while the rest of the module is a release build.

For the TensorRT-LLM logger this matters because of:

```cpp
// csrc/nv_internal/include/tensorrt_llm/common/logger.h
#ifndef NDEBUG
  Level const DEFAULT_LOG_LEVEL = DEBUG;
#else
  Level const DEFAULT_LOG_LEVEL = INFO;
#endif
```

With `NDEBUG` missing on the host side, every prebuilt
`flashinfer-jit-cache` wheel ships with `Logger::level_ = DEBUG (10)`.
On Hopper this turns each MoE forward pass into a stream of
`[TensorRT-LLM][DEBUG] ... sm90_generic_mixed_moe_gemm_kernelLauncher
...` lines from the OSS CUTLASS kernel dispatcher. Verified by reading
the data-section initializer of `Logger::Logger()` in the released
`flashinfer-jit-cache==0.6.10+cu130`
`fused_moe_{90,100,103,120,trtllm_sm100}.so` — all five start `Logger`
with `DEFAULT_LOG_LEVEL=10` and `level_=10`, even though the same wheels
carry no `.debug_*` sections (i.e. they are otherwise release-built).

The fix is one line: also append `-DNDEBUG` to the host `cflags` when
not in debug mode. The `flashinfer-jit-cache` wheel build picks this up
automatically and the prebuilt logger flips back to `INFO`.

## 🔍 Related Issues

Initially this bug was observed during integration of FI v0.6.10 into
vLLM: [[CI/Build] Bump flashinfer to v0.6.10
#41711](https://github.com/vllm-project/vllm/pull/41711).
There is a CI job log failure due to this issue:
[buildkite/ci/pr/distributed-tests-2-gpus-h100](https://buildkite.com/vllm/ci/builds/64532#019df966-e67d-4c27-af0e-76b00bc496e5).

Surfaced while debugging a downstream CI step that produced a 2.9 GB log
dominated by TRT-LLM debug prints from `fused_moe_90.so`. No FlashInfer
issue tracking this yet — happy to file one alongside this PR if useful.

## 🚀 Pull Request Checklist

### ✅ Pre-commit Checks

- [x] I have installed `pre-commit` by running `pip install pre-commit`.
- [x] I have installed the hooks with `pre-commit install`.
- [x] I have run the hooks manually with `pre-commit run --all-files`
and fixed any reported issues.

## 🧪 Tests

- [x] Tests have been added or updated as needed.
- [x] All tests are passing (`pytest tests/test_jit_cpp_ext.py`).

Two regression tests added in `tests/test_jit_cpp_ext.py`, mirroring the
existing `test_debug_jit_uses_sccache_compatible_nvcc_device_debug_flag`
style:

```
pytest tests/test_jit_cpp_ext.py -v
```

```
test_release_jit_propagates_ndebug_to_host_cflags PASSED
test_debug_jit_does_not_propagate_ndebug          PASSED
```

The first asserts that a release build
(`FLASHINFER_JIT_DEBUG`/`FLASHINFER_JIT_VERBOSE` unset) puts `-DNDEBUG`
in **both** `spec.extra_cflags` and `spec.extra_cuda_cflags`. The second
locks in symmetry: with `FLASHINFER_JIT_DEBUG=1` neither list contains
`-DNDEBUG`. Without the fix, the first test fails on `assert "-DNDEBUG"
in spec.extra_cflags`.

## Reviewer Notes

Single-line behavior change in `flashinfer/jit/core.py`. No effect on
debug builds. Prebuilt wheels rebuilt from this commit will pick up the
change automatically — no schema/version bump needed.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **New Features**
* JIT-compiled code now includes optimized compilation flags in release
mode for improved performance.

* **Tests**
* Added test coverage for proper compilation flag handling between debug
and release build modes.

[![Review Change
Stack](https://storage.googleapis.com/coderabbit_public_assets/review-stack-in-coderabbit-ui.svg)](https://app.coderabbit.ai/change-stack/flashinfer-ai/flashinfer/pull/3278)

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Artem Perevedentsev <aperevedents@nvidia.com>