fix(windows): preserve provider launch environment and test native shims

[dviejokfs]

Windows provider launches need system/profile variables and npm shim resolution while preserving explicit argv and excluding unrelated credentials. This change preserves a case-insensitive environment allowlist, resolves bare Windows launchers against the effective PATH, and suppresses launch/cleanup console windows.

Native fixtures exercise JSON, Unicode, spaces, shell metacharacters, environment-expansion syntax, exact stdin, explicit environment overlays, nonzero exits and cancellation. Windows tests run Claude/Codex/OpenCode .cmd fixtures through the same launcher and reject CR/LF arguments before execution. Rust 1.88 includes the batch-escaping fix introduced in 1.77.2; no raw_arg is used.

Also forwards Fleet developer instructions to Codex thread start/resume, probes OpenCode through /global/health, and only treats deliberate attached-server termination as successful after a terminal protocol event. Verification exposed an early-exit race in extension commands: stdin and stdout now run concurrently under the existing deadline so a rejecting child's stderr is preserved instead of being replaced by BrokenPipe.

Validation:

• Local macOS SDK suite: 302 passed, 1 existing ignored test.
• Strict all-target/all-feature Clippy and cargo check passed.
• New early-rejection regression fails before the fix and passes afterward.
• Source security review passed; cross-platform GitHub CI verifies native Windows/macOS/Linux execution.

Authenticated provider journeys, Windows OS services and Job Object ownership are outside these fixtures. SSH password authentication remains Unix-only. The cancellation fixture proves immediate-child termination, not a full Windows descendant-process lifecycle.