Gate api-simulator dev dependency to Linux only #954

Problem

The api-simulator dev dependency is declared unconditionally in pyproject.toml, causing macOS CI to fail on stable-targeting PRs (e.g. PR #945). The Rust crate's Python bindings (api-simulator-py) request features = ["full"], which includes process-pty — a feature with an explicit compile_error! on non-Linux platforms.

api-simulator is a development/testing tool only — no stable user would ever need it.

Root Cause

• pyproject.toml line 52: "api-simulator" in [project.optional-dependencies] dev with no platform marker
• crates/api-simulator-py/Cargo.toml: api-simulator = { path = "../api-simulator", features = ["full"] }
• crates/api-simulator/Cargo.toml: full includes process-pty
• crates/api-simulator/src/process/mod.rs:17: compile_error! on non-Linux

Fix

Add a sys_platform == 'linux' marker to the api-simulator dev dependency in pyproject.toml:

dev = [
    "api-simulator; sys_platform == 'linux'",
    ...
]

Then regenerate the lockfile with uv lock.

Any tests importing api-simulator should already be skipped on non-Linux (imports are lazy and env-var-gated).

Context

• Investigation report: .autoskillit/temp/investigate/investigation_macos-ci-failure-pr945_2026-04-15_143000.md
• Blocking PR: test #945 (main → stable promotion)