test: move handler handler tests to testcontainers db

[EItanya]

Summary

• switch HTTP handler tests from the in-memory fake database client to the shared Postgres testcontainers setup
• add a shared database-backed test helper for the handlers test package
• delete the unused fake database implementation

Testing

• go test ./core/internal/httpserver/handlers/...

[Copilot]

Pull request overview

This PR updates the handlers_test suite to exercise HTTP handlers against the real Postgres-backed database.Client (via the shared testcontainers Postgres harness), and removes the now-unused in-memory fake DB implementation.

Changes:

• Switch handler tests to use a shared testcontainers Postgres database via a new setupTestDBClient helper and TestMain.
• Update tests to use real DB behaviors/APIs (e.g., tool refresh, task storage/response types).
• Delete the go/core/internal/database/fake in-memory client implementation.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
go/core/internal/httpserver/handlers/toolservers_test.go	Uses setupTestDBClient and real DB tool discovery refresh instead of fake tool insertion.
go/core/internal/httpserver/handlers/sessions_test.go	Uses setupTestDBClient; updates task creation/assertions to use protocol.Task with real DB storage.
go/core/internal/httpserver/handlers/memory_test.go	Uses setupTestDBClient to back memory handler tests with Postgres/pgvector.
go/core/internal/httpserver/handlers/database_test.go	Adds TestMain + setupTestDBClient for shared containerized Postgres and per-test truncation.
go/core/internal/httpserver/handlers/agents_test.go	Switches to setupTestDBClient in handler setup (removes fake DB client usage).
go/core/internal/database/fake/client.go	Removes the unused in-memory fake DB client implementation.

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