fix: set preferred transport on agent cards

[gnanirahulnutakki]

Summary

• set PreferredTransport to JSONRPC when the controller builds A2A agent cards
• add regression coverage for GetA2AAgentCard so controller-generated cards keep that field populated
• keep the change limited to the translator path described in [BUG] Agent card missing preferredTransport breaks a2a-go client #1684

Testing

• /opt/homebrew/bin/go test ./core/internal/controller/translator/agent -run TestGetA2AAgentCard

Fixes #1684

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Sets PreferredTransport on controller-generated A2A agent cards to maintain a populated transport field and adds regression coverage to prevent future regressions (per #1684).

Changes:

• Populate PreferredTransport with "JSONRPC" in GetA2AAgentCard.
• Add a regression assertion in TestGetA2AAgentCard to verify PreferredTransport is set.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
go/core/internal/controller/translator/agent/utils.go	Sets PreferredTransport during agent card construction.
go/core/internal/controller/translator/agent/utils_test.go	Adds regression assertion verifying PreferredTransport remains populated.

Comments suppressed due to low confidence (1)

go/core/internal/controller/translator/agent/utils.go:1

• PreferredTransport is being set via a newly introduced magic string ("JSONRPC"). To reduce drift/typos and make future updates easier, consider defining a package-level constant (or reusing an existing shared constant/enum if one exists in the codebase) and referencing that instead of repeating a raw string literal.

package agent

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[Copilot]

This pattern works, but it’s more idiomatic and provides clearer failure behavior to use require.NotNil(t, card.PreferredTransport) and then assert the value on the next line (no conditional needed). That keeps the test intent straightforward and avoids silently skipping the value assertion after a nil failure.

[gnanirahulnutakki]

Good call. I tightened this in the follow-up push: the test now uses require.NotNil(t, card.PreferredTransport) and then asserts the JSON-RPC value on the next line.

[gnanirahulnutakki]

I pushed a follow-up commit after checking the failing CI job more closely. The go-unit-tests failure was real: adding preferredTransport changed the generated ADK translator output, so the golden fixtures needed to be updated alongside the controller/unit test change.

What changed in the follow-up:

• switched the controller helper to use the shared JSON-RPC transport constant at the string boundary expected by server.AgentCard
• tightened the regression test to require.NotNil(...) before asserting the value
• regenerated the translator golden outputs so the new preferredTransport field and resulting config-hash changes are captured intentionally

Local verification on the rebased branch:

• go test ./core/internal/controller/translator/agent -count=1
• go test -race ./core/internal/controller/translator/agent -count=1