[fix](cli): detect SGLANG_DSV4_2604_SUBMODE conflict before launch

[devangpratap]

Fixes #2013

What does this PR do?

SGLANG_DSV4_2604_SUBMODE=2604B is a DeepSeek-V4-Flash submode flag that
injects swiglu_limit=10.0 into MoE layers. kt-kernel only supports this on
the MXFP4 backend. If it is left in the environment from a previous MXFP4
launch and the user switches to another method (e.g. AMXINT4), startup crashes
with a cryptic ValueError deep in model loading -- after 150+ seconds of weight
loading has already completed.

This PR adds two layers of detection inside the CLI:

• kt run: after the subprocess env dict is fully assembled (shell +
  advanced.env + inference.env from kt config), check for the conflict and
  exit immediately with a clear error message before sglang is ever launched.
• kt doctor: surface SGLANG_DSV4_2604_SUBMODE as a WARNING row in the
  diagnostics table whenever it is set to 2604B, so users can catch the
  conflict proactively.

Note: users who invoke python -m sglang.launch_server directly are not
affected by the kt run guard, but the existing error message in
experts.py already handles that path clearly.

Before submitting

• Did you read the contributor guideline?
• Did you write any new necessary tests?

[gemini-code-assist]

Code Review

This pull request introduces validation and diagnostic checks for the SGLANG_DSV4_2604_SUBMODE environment variable to prevent startup crashes when the chosen kt-method is not MXFP4. In doctor.py, a diagnostic check is added to surface this environment variable. In run.py, a fail-fast check is introduced to exit early if the conflicting environment variable is set. Feedback from the review suggests: 1) In doctor.py, only surface the environment variable row if it has a non-empty value (instead of just checking is not None) to avoid noise. 2) In run.py, guard against kt_method being None or empty to prevent an AttributeError when calling .upper().

[gemini-code-assist]

If SGLANG_DSV4_2604_SUBMODE is set to an empty string in the environment, checking dsv4_submode is not None will still add a row to the doctor diagnostics table with an empty value. To avoid unnecessary noise in the diagnostics output, it is better to only surface the row if the variable has a non-empty value.

Suggested change

	dsv4_submode = os.environ.get("SGLANG_DSV4_2604_SUBMODE")
	if dsv4_submode is not None:
	dsv4_submode = os.environ.get("SGLANG_DSV4_2604_SUBMODE")
	if dsv4_submode:

[gemini-code-assist]

To ensure robust defensive programming, we should guard against the possibility of kt_method being None or empty. If kt_method is None, calling kt_method.upper() will raise an AttributeError. Adding a check for not kt_method prevents this crash and correctly identifies the conflict since a missing method is not MXFP4.

Suggested change

	dsv4_submode = env.get("SGLANG_DSV4_2604_SUBMODE", "")
	if dsv4_submode == "2604B" and kt_method.upper() != "MXFP4":
	dsv4_submode = env.get("SGLANG_DSV4_2604_SUBMODE", "")
	if dsv4_submode == "2604B" and (not kt_method or kt_method.upper() != "MXFP4"):

[devangpratap]

Applied both suggestions — empty-string guard in doctor.py and None-check before .upper() in run.py.