path-uri: remove legacy path deserialization

[anp-oai]

Why

I'd originally added PathUri legacy path deserialization thinking we'd want it for having PathUri in public app-server APIs. Since then we've added LegacyAppPathString to handle the messy conversions that we need for backcompat. It's confusing for PathUri to support deserializing legacy paths when we don't yet want to actually expose app-server callers or rollout storage to the new URI format.

Stacked on top of #29472 to avoid breaking compatibility in case those types ended up stored somewhere for someone.

What changed

• Parse deserialized PathUri values exclusively as valid file: URIs.
• Replace legacy acceptance coverage with rejection coverage for top-level filesystem paths and sandbox working directories.
• Serialize CWDs in hand-built exec-server process requests as PathUri values.

[anp-oai]

@codex review

[chatgpt-codex-connector]

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[anp-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🎉

Reviewed commit: f832435bda

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