Adopt error API to convey server errors to the client

[Hardikrepo]

Summary

• Adds a PIRServiceError type that serializes the new Apple_SwiftHomomorphicEncryption_Api_Pir_V1_Error protobuf message (from Adding Error API swift-homomorphic-encryption-protobuf#21) as the HTTP response body, following the same HTTPResponseError pattern already used for HeError/PirError.
• Rewires the four error paths in PIRServiceController.queries() to throw PIRServiceError instead of plain HTTPError:
  • Unknown usecase / unparseable request -> invalidRequest
  • Missing evaluation key -> evaluationKeyNotFound
  • Stale/unknown config id -> configVersionNotFound, now with a fresh ConfigResponse embedded in the error body, so the client can retry immediately without a second config round trip (this is what Document database reloading functionality #94's linked comment asked for).
• Factored the config-building logic in config() into a shared configResponse(usecases:existingConfigIds:context:) helper reused by both endpoints.

Fixes #102.

Test plan

• CI (build + swift test)
• Manual smoke test: send a queries request with a stale configurationHash and confirm the response body decodes as Apple_SwiftHomomorphicEncryption_Api_Pir_V1_Error with configVersionNotFound.configResponse populated

[karulont]

Thank you for the PR.

I expect there to be unit tests that cover this change.

[karulont]

This is a new file that was authored most likely in 2026, not during 2024 and then later changed in 2025.

Suggested change

	// Copyright 2024-2025 Apple Inc. and the Swift Homomorphic Encryption project authors
	// Copyright 2026 Apple Inc. and the Swift Homomorphic Encryption project authors

[karulont]

File name should probably be PIRServiceError, because this file contains the definition of it.
And the location of it should not be under Extensions.