Join Response.String() messages with a newline like .NET ConcatText

[PratikDhanave]

What

Response.String() flattened the text of every message into a single strings.Builder with no separator. For a two-message assistant turn (foo, Bar) it returned fooBar. It now joins each non-empty per-message text with a newline, yielding foo\nBar, and skips empty messages so no double newline is produced. Text glued within a single message (multiple TextContent items) is unchanged.

Why (.NET parity)

In microsoft/agent-framework, AgentResponse.Text is backed by messages.ConcatText(). ConcatText(IList<ChatMessage>) documents that "A newline separator is added between each non-empty piece of text" and uses AppendLine between non-empty messages. The Go port glued messages together instead, so the same two-message turn produced fooBar in Go vs foo\nBar in .NET.

The per-message join was already correct: msg.String() == message.Contents.Text(), the analog of ChatMessage.Text. This change reuses msg.String() and only fixes the cross-message separator.

This also aligns the agent-as-tool result: agenttool's FuncTool returns resp.String() as the tool output, the analog of .NET AsAIFunction returning response.Text.

Testing

Added TestResponse_String in agent/response_test.go (black-box, package agent_test) covering: two single-text messages join with a newline; an empty middle message is skipped without a double newline; multiple TextContent items within one message stay glued. go build ./..., go vet ./agent/..., and go test ./agent/... (plus ./tool/agenttool/...) pass.

[Copilot]

Pull request overview

This PR aligns agent.Response.String() output with the .NET ConcatText / AgentResponse.Text behavior by inserting a newline between each non-empty message’s text, instead of concatenating message texts with no separator.

Changes:

• Update Response.String() to join each non-empty msg.String() with \n and skip empty messages.
• Add black-box tests covering newline joining, skipping empty messages without double newlines, and preserving within-message text concatenation across multiple TextContent items.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
agent/response.go	Changes Response.String() to newline-separate non-empty message text using msg.String() (which delegates to message.Contents.Text()).
agent/response_test.go	Adds TestResponse_String to validate cross-message newline joining and empty-message skipping while keeping within-message concatenation unchanged.

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