Decouple plugin manifest path resolution

[jif-oai]

Why

Plugin manifests use the same schema whether the package lives on the host or in an executor. Only the path representation differs: host callers need native Path inputs and AbsolutePathBuf outputs, while executor callers need PathUri throughout.

Maintaining separate parsing or resolver implementations would duplicate the manifest rules and allow them to drift. This PR instead makes URI-native resolution the single parsing path and keeps host conversion at the boundary.

What changed

• Make parse_plugin_manifest_uri the shared manifest parser and resolve every path-bearing field as PathUri.
• Keep the existing host entrypoint as a thin adapter: convert its native root and manifest path to PathUri, run the shared parser, then map resources back to AbsolutePathBuf.
• Expose PluginManifest::try_map_resources so callers can convert the generic resource type without duplicating manifest construction.
• Resolve relative manifest paths using the root URI's convention: backslashes are separators for Windows roots and ordinary filename characters for POSIX roots.
• Apply lexical containment after URI resolution, rejecting absolute paths and parent traversal outside the plugin root.
• Make encoded backslashes fail containment only for Windows URIs; encoded / remains unsafe for every convention.
• Use a host-native synthetic root for marketplace fallback manifests so the host adapter also works on Windows.

host Path --------> PathUri --\
                              +--> one manifest parser --> PluginManifest<PathUri>
executor PathUri -------------/

host result: PluginManifest<PathUri> --> PluginManifest<AbsolutePathBuf>

Existing host manifest behavior is preserved; #28918 is the first executor consumer.

Verification

• just test -p codex-utils-path-uri
• just test -p codex-plugin
• just test -p codex-core-plugins

Stack

1. path-uri: add lexical containment #29614 — add lexical PathUri containment.
2. This PR — share URI-native manifest path resolution.
3. Make selected plugin roots URI-native #28918 — keep selected plugin roots and resources URI-native.
4. Load executor skills without host path conversion #29626 — load executor skills without host path conversion.
5. Keep executor plugin MCP paths URI-native #29628 — resolve executor MCP working directories without host path conversion.

[chatgpt-codex-connector]

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