feat(app-server): expose filesystem metadata size

[nornagon-openai]

Why

Codex Apps adapts app-server filesystem responses to its execution-host interface, which needs file sizes from metadata queries.

What

Add sizeBytes to the fs/getMetadata response and map the existing filesystem metadata size through app-server. Update the protocol fixtures, API documentation, and metadata behavior coverage.

This PR is intentionally limited to file size metadata. Symlink semantics and exclusive copies are split into independent changes.

Validation

• just write-app-server-schema
• just test -p codex-app-server-protocol
• just test -p codex-app-server fs_get_metadata
• just fmt

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Use an exact wire type for file sizes

When fs/getMetadata is called on a sparse or otherwise very large file whose length exceeds Number.MAX_SAFE_INTEGER, this new u64 is exposed to generated TypeScript clients as a number, so JS clients lose low bits before using the size for range/copy decisions. Since the field is meant to carry filesystem metadata, expose an exact representation (or reject/cap unsafe values) rather than advertising every u64 as a JS number.

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