[P2] Public testing kit: scriptable fake LLM provider, fake memory, and fake tools (v1beta/agenttest)

[kunalkushwaha]

Problem

Users have no supported way to unit-test agents without hitting a real LLM:

• A mock provider exists (ProviderTypeMock, internal/llm/factory.go:26,477-484; internal/llm/mock_adapter.go) and is technically reachable via provider = "mock", but it is undocumented, internal-flavored, and not scriptable (fixed responses only).
• No fake memory, fake tool, or stream-assertion helpers are exported. The repo's own tests live in test/ and internal/ and aren't reusable by consumers.
• Frameworks users will compare against (Genkit, LangChainGo) ship test helpers; for a framework whose selling point is production Go, a testing story is table stakes.

Proposed change

Add a v1beta/agenttest package:

1. Scriptable fake provider: enqueue responses, including tool-call turns and streamed chunk sequences; assert on received messages/params.

   fake := agenttest.NewProvider().
       ExpectUserContains("weather").
       RespondWithToolCall("get_weather", args).
       ThenRespond("It's sunny in Pune.")

2. Fake memory (in-memory, inspectable store/query log) and fake tools (canned results, call recording).
3. Stream test helpers: collect chunks, assert ordering/types (ChunkTypeDelta, ChunkTypeToolCall, ChunkTypeDone).
4. Document in a "Testing your agents" guide under docs/v1beta/.

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From docs/AGNOSTIC_FRAMEWORK_ANALYSIS.md (branch claude/go-agnostic-framework-analysis-ekyl3a), §5 & §6.