feat(forestadmin-client): add fallback to ForestHttpApi for partial server interfaces

[Scra3]

Summary

• Add withDefaultImplementation function that wraps partial ForestAdminServerInterface with a Proxy
• Methods not implemented in the custom interface will fallback to ForestHttpApi default implementation
• buildApplicationServices now accepts Partial<ForestAdminServerInterface> instead of full interface

Problem

After PR #1411 (centralize HTTP calls), partial implementations like ForestAdminServerSwitcher in cloud-lambda-layers would fail when calling methods they didn't implement (e.g., checkSchemaHash, postSchema).

The comment in ForestAdminServerSwitcher stated:

"All methods in ForestAdminServerInterface are optional. When not implemented, the agent's default implementation is used."

But this behavior was never actually implemented - there was no fallback mechanism.

Solution

Use a Proxy to intercept method calls on the provided interface:

1. If the method exists on the custom interface → use it
2. If not → fallback to the default ForestHttpApi implementation

Test plan

• [x] All existing tests pass (yarn workspace @forestadmin/forestadmin-client test)
• [x] Lint passes (yarn workspace @forestadmin/forestadmin-client lint)
• [x] Build passes (yarn workspace @forestadmin/forestadmin-client build)

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