Fix Windows marketplace paths and shell command strings

[jgershen-oai]

Summary

• treat Windows backslash-relative/rooted marketplace sources as local paths
• keep the local marketplace config test on structured TOML access so Windows backslashes are not interpreted through indexing/string escapes
• add a test helper for shell_command responses that accepts an already-constructed raw command string, preserving Windows PowerShell quoting semantics

Stack

• Stacked on Make thread unsubscribe test deterministic #18000 / starr/deterministic-thread-unsubscribe
• Extracted from the closed broad CI-stabilization PR [codex] Stabilize Windows Bazel test flakes #17895
• Intentionally excludes the timeout/concurrency/test-sweep changes from [codex] Stabilize Windows Bazel test flakes #17895

Validation

• just fmt
• cargo test -p codex-core marketplace_add
• cargo test -p codex-cli marketplace_add
• cargo test -p codex-app-server suite::v2::thread_unsubscribe::thread_unsubscribe_during_turn_keeps_turn_running -- --exact
• git diff --check