[codex] make PathUri::from_abs_path infallible

[anp-oai]

Why

PathUri::from_abs_path can fail for absolute paths that do not have a normal file: URI representation, forcing filesystem call sites to handle a conversion error even though the original path can be preserved losslessly.

What

Make from_abs_path infallible and migrate its callers. Unrepresentable paths use file:///%00/bad/path/<base64>, encoding Unix bytes or Windows UTF-16LE; to_abs_path validates and decodes that fallback. The leading encoded null reserves a namespace that cannot collide with a real Unix or Windows path, and fallback URIs remain opaque to lexical path operations.

Validation

Added path-URI coverage for Unix null and non-UTF-8 paths, Windows device/verbatim and non-Unicode paths, serialization, malformed fallbacks, opaque lexical operations, invalid native payloads, and literal /bad/path collision resistance.

[chatgpt-codex-connector]

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