[DRAFT] MAINT Separate server launch, readiness, and cleanup state (#2415)

[Zhenyu Wang (zhenyu-02)]

Description

Closes #2415.

This refactors ServerLauncher.start_async into three internal seams while preserving it as the public orchestrator:

• a pure launch plan for validation, command construction, and Windows/Unix process settings;
• explicit startup state for process ownership, PID persistence, readiness, timeout, diagnostics, and cleanup results;
• one idempotent, cancellation-safe cleanup path shared by launch failure, readiness failure, timeout, cancellation, and stop().

The process-group, detached-launch, PID-record, log, and public return/exception behavior remain unchanged. Focused tests cover platform-specific planning, invalid timeouts, cancellation during spawn/PID persistence/health polling/cleanup, repeated cancellation, readiness success, crash diagnostics, timeout cleanup, and cleanup failure retention.

Tests and Documentation

• pytest -q tests/unit/cli/test_server_launcher.py — 67 passed, 1 skipped
• pytest -q tests/unit/cli — 430 passed, 1 skipped
• ruff check pyrit/cli/_server_launcher.py tests/unit/cli/test_server_launcher.py
• ruff format --check pyrit/cli/_server_launcher.py tests/unit/cli/test_server_launcher.py
• ty check pyrit/cli/_server_launcher.py tests/unit/cli/test_server_launcher.py
• python -m compileall -q pyrit/cli/_server_launcher.py tests/unit/cli/test_server_launcher.py
• git diff --check
• diff coverage: 100% of 169 changed executable lines

No public API or documentation behavior changed. JupyText: N/A.

[Zhenyu Wang (zhenyu-02)]

@microsoft-github-policy-service agree

[Roman Lutz (romanlutz)]

Is this meant to be a draft PR or ready for review? Zhenyu Wang (@zhenyu-02)