fix: preserve unknown RPC return types

[nanookclaw]

Summary

Fixes #5200.

Rpc.Result<R> currently falls through to never when R is unknown. The same failure affects concrete object types containing unknown fields, such as { bar: unknown[] }, even though those values can be carried over the RPC boundary.

This adds an unknown extends T branch to Serializable<T>. The guard accepts unknown only when it is the type being checked; known types continue through the existing recursive serializability checks. Existing rejection of known non-serializable values, including ReadableStream<string>, is preserved. Type-level coverage now exercises both the top-level and nested-unknown cases.

Verification

• git diff --check
• Focused TypeScript sanity checks against the exact RPC definitions, covering unknown acceptance and ReadableStream<string> rejection.
• The full Bazel type target was not run because this checkout environment has no Bazel binary or installed repository dependencies.

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