[codex] Reject unsupported remote pairing protocols

[StiensWout]

Summary

• Reject unsupported remote backend protocols during remote pairing resolution.
• Keep accepted backend protocols to http:, https:, ws:, and wss: while preserving bare-host https:// defaults.
• Add focused coverage for unsupported direct pairing URLs, hosted backend hosts, and direct host inputs.

Root cause

Remote pairing URL parsing accepted any syntactically valid URL protocol. Unsupported schemes such as ftp: could pass through direct pairing resolution or normalized backend host resolution and produce backend base URLs that the app cannot use.

Impact

Invalid remote pairing targets now fail early with the existing structured errors. Direct unsupported pairing URLs throw RemotePairingUrlInvalidError; unsupported hosted or direct backend host inputs throw RemoteBackendUrlInvalidError with their existing source metadata preserved.

Validation

• PATH="$HOME/.vite-plus/bin:$PATH" vp test packages/shared/src/remote.test.ts
• PATH="$HOME/.vite-plus/bin:$PATH" vp check
• PATH="$HOME/.vite-plus/bin:$PATH" vp run typecheck