feat(profiling): add extra_nsys_args for optional nsys CLI flags

[zhengd-nv]

PR description

Summary

Adds an optional profiling.extra_nsys_args field (list[str] | null) so users can append extra arguments to the nsys profile wrapper for both the default (SGLang) and TRTLLM worker command prefixes, without changing the fixed baseline flags.

Behavior

• Placement: Extra tokens are inserted immediately before -o <output> so output path and downstream behavior stay predictable.
• Modes: Applies when profiling type is nsys or nsys-time (same get_nsys_prefix / _get_nsys_prefix_trtllm paths as existing nsys support).
• TRTLLM: The TRTLLM-specific prefix also honors extra_nsys_args with the same ordering relative to -o.
• Dynamo frontend: Existing --trace-fork-before-exec=true insertion for frontend_type="dynamo" remains unchanged relative to -o.

Configuration

YAML example:

profiling:
  type: "nsys"
  extra_nsys_args: ["--stats=true", "--trace=osrt"]
  # ... prefill/decode/aggregated as required by mode

Documented in docs/profiling.md and docs/config-reference.md.

Tests

• tests/test_profiling.py: asserts extra args appear in the prefix and before -o for the default path and for backend_type="trtllm".

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@ef7d598). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #59   +/-   ##
=======================================
  Coverage        ?   70.52%           
=======================================
  Files           ?       60           
  Lines           ?     6325           
  Branches        ?        0           
=======================================
  Hits            ?     4461           
  Misses          ?     1864           
  Partials        ?        0           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.