app-server: use native path strings for command actions

[anp-oai]

Why

Prepare command-action read paths to cross the app-server boundary without assuming the app-server host path syntax. This is stacked on #27819.

What

• Represent CommandAction::Read.path as NativePathString and resolve it to PathUri with an explicit source convention.
• Add host-independent POSIX, Windows drive, and UNC parsing, including dot-segment normalization.
• Update app-server builders, transitional native-host consumers, generated schemas, and API documentation.

Validation

• Added cross-platform native-path round-trip and command-action wire tests.
• codex-utils-path-uri, codex-app-server-protocol, codex-app-server, and codex-analytics suites pass.
• Bazel lockfile validation passes.
• The TUI suite ran 2,825 tests: 2,813 passed and 12 unrelated guardian/title/status-preview baseline tests failed reproducibly.