fix(mcp): include SandboxAgents in list_agents and invoke_agent

[anxkhn]

What

list_agents, the kagent://agents MCP resource, and invoke_agent now return
SandboxAgents (substrate-backed agents) in addition to declarative Agents. A
fully ready SandboxAgent was previously invisible to all three, even though it
shows up in the REST /agents API and the UI.

Why

MCPHandler.listReadyAgents (go/core/internal/mcp/mcp_handler.go) had two
compounding faults:

1. It listed only v1alpha2.AgentList, so SandboxAgent CRs were never
   enumerated at all.
2. It gated readiness on the string literal condition.Reason == "DeploymentReady".

A SandboxAgent reports its Ready condition with reason WorkloadReady, while
a declarative Agent uses DeploymentReady:

• reconcileSandboxAgentStatus sets Reason = AgentReadyReasonWorkloadReady
  when the workload is ready (reconciler.go).
• reconcileAgentStatus sets Reason = AgentReadyReasonDeploymentReady
  (reconciler.go).

So a SandboxAgent with Accepted=True and Ready=True (reason WorkloadReady)
was dropped by both faults. listReadyAgents feeds handleListAgents (the
list_agents tool) and readAgentsResource (the kagent://agents resource), and
invoke_agent relies on the same readiness notion, so such an agent was reported
as "not found or not ready" over MCP while being fully usable via the REST API.

The correct, two-reason, interface-based readiness check already exists elsewhere
in the codebase and is what this change reuses:

• isAgentReady in the A2A registrar (a2a/a2a_registrar.go) accepts both
  AgentReadyReasonDeploymentReady and AgentReadyReasonWorkloadReady over the
  AgentObject interface.
• The REST agents handler (httpserver/handlers/agents.go) does the same.

Fix

• List both v1alpha2.AgentList and v1alpha2.SandboxAgentList, and evaluate
  readiness through a small readyAgentSummary(v1alpha2.AgentObject) helper that
  operates on the shared AgentObject interface.
• The helper accepts both reconciler.AgentReadyReasonDeploymentReady and
  reconciler.AgentReadyReasonWorkloadReady, and uses the typed condition-type
  constants (v1alpha2.AgentConditionTypeReady / ...Accepted) with
  metav1.ConditionTrue, matching the A2A registrar and the REST handler instead
  of re-implementing the check with hardcoded strings.
• Updated the list_agents tool and kagent://agents resource descriptions from
  "accepted + deploymentReady" to "accepted + ready" now that both workload types
  are covered.

No CRD/API types change, so no code generation is required.

Tests

Added TestListReadyAgents_IncludesSandboxAgents in
go/core/internal/mcp/mcp_handler_test.go (table of fake objects via the
controller-runtime fake client already used in this file). It asserts that:

• a ready declarative Agent (DeploymentReady) is listed,
• a ready SandboxAgent (WorkloadReady) is listed,
• a not-ready SandboxAgent is excluded, and
• the SandboxAgent's description is surfaced.

The test fails against the current code (the SandboxAgent is absent and its
description is empty) and passes with the fix. Revert the mcp_handler.go hunks
to reproduce.

How to verify

cd go
go build ./core/...
go test -race ./core/internal/mcp/...   # incl. TestListReadyAgents_IncludesSandboxAgents
make -C go lint                         # from repo root; 0 issues

Scope

Limited to the MCP agent-listing path. It reuses the existing readiness semantics
(the same two Ready reasons the REST API and A2A registrar already honor) rather
than introducing new behavior, so declarative Agent results are unchanged and
only genuinely ready SandboxAgents are added.

Checklist

• Commit is DCO signed-off (git commit -s).
• go build ./core/..., go test -race ./core/internal/mcp/..., and
  make -C go lint all pass.
• Unit test added; fails without the fix, passes with it.
• Diff limited to go/core/internal/mcp/mcp_handler.go and its test.

[Copilot]

Pull request overview

This PR updates the MCP agent-discovery path so that list_agents, the kagent://agents resource, and the readiness filtering they rely on include both declarative Agent CRs and substrate-backed SandboxAgent CRs, using the shared v1alpha2.AgentObject interface and the same “Accepted + Ready” semantics already used by the REST /agents endpoint and the A2A registrar.

Changes:

• Expand MCPHandler.listReadyAgents to list both AgentList and SandboxAgentList.
• Replace hard-coded condition-string readiness checks with a shared readyAgentSummary(v1alpha2.AgentObject) helper that recognizes both DeploymentReady and WorkloadReady ready reasons.
• Add a regression unit test ensuring ready SandboxAgents are included and their descriptions are surfaced.

Reviewed changes

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

File	Description
go/core/internal/mcp/mcp_handler.go	Lists both Agent and SandboxAgent CRs and applies interface-based readiness checks for MCP agent discovery.
go/core/internal/mcp/mcp_handler_test.go	Adds a regression test covering ready vs. not-ready SandboxAgent inclusion and description propagation.

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