Support co-located VFS providers and request-scoped operations

[DeBaschdi]

Summary

• Allow a WebExtension to register and consume a provider from the same add-on without relying on management discovery.
• Acknowledge connection-cache updates before setup or removal returns.
• Bind storage grants and setup completion to the verified runtime consumer and originating port.
• Add request-scoped cancellation for read, list and picker operations while retaining the existing bulk abort API.

Why

The toolkit currently discovers only other extensions, so an add-on cannot expose a provider and consume it through the same VFS API.

Provider setup can also return before the picker cache contains the new connection. Caller-provided IDs must not establish identity, and provider-wide cancellation can interrupt unrelated operations.

Compatibility

The API version and existing operation payloads remain unchanged.

Existing external providers continue to work with the updated client. The provider still accepts the original API 1.3 cancel message without its own request ID, so older clients remain compatible. New clients add a request ID only for correlation and request ownership.

Co-located provider support is opt-in through registerLocalProvider().

Verification

• Client and provider syntax checked for every commit
• Example client and provider add-ons built successfully
• Co-located registration, setup and connection removal tested
• Old/new API 1.3 cancellation compatibility tested
• Cross-consumer grant isolation and request-scoped cancellation tested