[dotnet-port-api] Port message source attribution helpers

[Michelle Clayton (michelle-clayton-work)]

Summary

Port the small .NET message-source attribution helper surface to Go by adding message.Message helpers for reading and applying request message source attribution.

This change adds (*message.Message).SourceType(), (*message.Message).SourceID(), and (*message.Message).WithSource(...), then reuses WithSource in the context-provider, history-provider, and middleware source-stamping paths. It also updates the feature comparison matrix to mark this helper surface as aligned.

Ported .NET PRs

• .NET: [BREAKING] Add ability to mark the source of Agent request messages and use that for filtering agent-framework#3540 - add request message source tagging/filtering support
• .NET: [BREAKING] Update message source code to match python. agent-framework#3805 - refine message source attribution to expose source type and source id helpers

Relevant upstream commits:

• ad0dac3c86201dc7dc1cd4866ba7e4502a910a0d
• 7dccf3a07bc5fd76004b85305f5121712e188b15

Breaking Changes

No.

Tests and Examples

• go test ./...
• Added message/message_test.go coverage for default source type, explicit source id/type access, and clone-vs-reuse behavior in WithSource
• No example changes were needed because the underlying source attribution workflow already existed; this PR ports the missing helper API surface

Notes

• This is a narrow API-parity PR focused on the helper surface called out in docs/dotnet-go-sdk-feature-comparison.md
• Broader history-provider reducer-trigger parity remains out of scope for this PR

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Closes #761

[Copilot]

🟢 Approval recommended

The helper methods are small, nil-safe, covered by focused unit tests, and the call-site refactors preserve prior behavior while reducing duplication.

Pull request overview

Ports the .NET message-source attribution helper API surface into the Go SDK by adding convenience helpers on message.Message, then refactors existing source-stamping call sites to reuse the new helper. This brings the Go SDK closer to documented API parity without altering the underlying source attribution model.

Changes:

• Added (*message.Message).SourceType(), SourceID(), and WithSource(...) helpers (nil-safe, clone-on-change behavior).
• Refactored context provider, history provider, and middleware source-stamping to use WithSource.
• Updated the .NET↔Go feature comparison matrix to mark source attribution helpers as aligned, and added unit tests for the new helpers.

File summaries

File	Description
agent/context.go	Replaces manual clone-and-stamp logic with msg.WithSource(...) for provided context messages.
agent/history.go	Replaces manual clone-and-stamp logic with msg.WithSource(...) for provided history messages.
agent/middleware.go	Uses WithSource when stamping middleware-generated messages with an empty source.
docs/dotnet-go-sdk-feature-comparison.md	Updates matrix row to reflect the new message.Message source attribution helpers.
message/message.go	Introduces SourceType(), SourceID(), and WithSource(...) helper methods on Message.
message/message_test.go	Adds tests for default source type behavior and WithSource clone-vs-reuse semantics.

Review details

• Files reviewed: 6/6 changed files
• Comments generated: 0
• Review effort level: Lite

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[github-actions]

Scope: public API, user-visible behavior

Changed Go contract: New exported message.Message methods SourceType() SourceType, SourceID() string, and WithSource(Source) *Message; agent/context.go, agent/history.go, and agent/middleware.go refactored to call WithSource instead of duplicating clone-and-tag logic inline (no behavior change to callers — same clone-only-when-changed semantics as before).

Upstream evidence reviewed:

• dotnet/src/Microsoft.Agents.AI.Abstractions/ChatMessageExtensions.cs — GetAgentRequestMessageSourceType(), GetAgentRequestMessageSourceId(), WithAgentRequestMessageSource(sourceType, sourceId) (from .NET: [BREAKING] Add ability to mark the source of Agent request messages and use that for filtering agent-framework#3540, #3805)
• dotnet/src/Microsoft.Agents.AI.Abstractions/AgentRequestMessageSourceType.cs — External (default), AIContextProvider, ChatHistory source type constants
• Go agent/context.go (SourceTypeContextProvider), agent/history.go (SourceTypeHistoryProvider), agent/middleware.go (SourceTypeMiddleware), message/message.go (SourceTypeExternal)

Result: aligned. The Go SourceType()/SourceID()/WithSource() trio is a faithful, idiomatic port of the .NET ChatMessageExtensions helpers: same default-to-external semantics, same "return original if source unchanged, else clone" behavior, and equivalent source-type taxonomy (external/context-provider/history-provider vs. .NET's External/AIContextProvider/ChatHistory, plus a Go-specific middleware source type consistent with existing Go call sites). The refactor of agent/context.go, agent/history.go, and agent/middleware.go to reuse WithSource is behavior-preserving (same clone-avoidance logic, just deduplicated). No unconditional enablement or default change was introduced — this only exposes helper methods around existing, already-active source-stamping behavior. Docs update in docs/dotnet-go-sdk-feature-comparison.md accurately reflects the new "Aligned" status.

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