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

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

🤖 Generated with Claude Code

[qltysh]

Coverage Impact

⬇️ Merging this pull request will decrease total coverage on main by 0.01%.

Modified Files with Diff Coverage (1)

Rating	File	% Diff	Uncovered Line #s
	packages/forestadmin-client/src/build-application-services.ts	92.3%	41
	Total	92.3%

🤖 Increase coverage with AI coding...

In the `feat/server-interface-fallback` branch, add test coverage for this new code:

- `packages/forestadmin-client/src/build-application-services.ts` -- Line 41

🚦 See full report on Qlty Cloud »

🛟 Help

• Diff Coverage: Coverage for added or modified lines of code (excludes deleted files). Learn more.

• Total Coverage: Coverage for the whole repository, calculated as the sum of all File Coverage. Learn more.

• File Coverage: Covered Lines divided by Covered Lines plus Missed Lines. (Excludes non-executable lines including blank lines and comments.)

  • Indirect Changes: Changes to File Coverage for files that were not modified in this PR. Learn more.