feat(app-server): complete filesystem host semantics

[nornagon-openai]

Why

Codex Apps is adapting app-server connections to its ExecutionHost interface. The filesystem API currently lacks several semantics that callers rely on, which forces adapters either to reject valid operations or silently change behavior.

What

• Add lstat-style metadata via fs/getMetadata.followSymlinks: false.
• Add sizeBytes to filesystem metadata.
• Add isSymlink to directory entries.
• Add atomic exclusive file copies via fs/copy.exclusive.
• Preserve already-exists errors across remote exec-server and expose structured { "code": "EEXIST" } app-server error data.
• Cover local and remote filesystem implementations and regenerate the JSON/TypeScript schemas.

Streaming file operations are intentionally separate and tracked by #27190.

Validation

• cargo test -p codex-app-server-protocol
• cargo test -p codex-app-server fs_ --no-fail-fast
• Targeted codex-exec-server local and remote tests for metadata, directory symlinks, and exclusive copies
• cargo check --tests -p codex-core
• just fix -p codex-file-system -p codex-exec-server -p codex-app-server-protocol -p codex-app-server
• just fmt