adcp/a2aclient: Go A2A buyer client with context retention

[bokelley]

Why

Port of adcontextprotocol/adcp-client-python#251 to Go. That PR added auto-retention of contextId and taskId across multi-turn A2A calls on the Python side. Without a Go equivalent, Go buyers can't speak A2A — half of AdCP's transport surface — and we can't run Go-side conformance against A2A agents.

Depends on a Go A2A server to test against (see #69). Can test against Python reference agents in the interim.

What

New package adcp/a2aclient mirroring the Python ADCPClient A2A adapter's API shape so the two stay legible together.

API

c := a2aclient.New(cfg,
    a2aclient.WithContextID("resume-or-self-named"), // seed for resume across restarts
)

resp, err := c.SendMessage(ctx, msg)  // auto-injects contextId + pending taskId

c.ContextID()        // current, server-authoritative
c.PendingTaskID()    // empty when last response was terminal
c.ResetContext()     // clear
c.ResetContext("id") // clear + seed

Retention rules

• Auto-adopt whatever contextId the server returns (supports ADK-style rewriting transparently).
• Retain taskId only while last response status ∈ {submitted, working, input-required, auth-required}.
• Clear taskId on terminal states.
• Default on — document the "one Client per conversation" rule on the type; last-write-wins under concurrent use.

Error behavior

• WithContextID / ResetContext(id) on a non-A2A-configured client return a typed error (symmetric writes).
• ContextID() / PendingTaskID() return empty strings when unset (lenient reads; safe in generic code).

Tests

• Unit: auto-retain, inject, reset, server rebinding, taskId retention in each non-terminal state, taskId clearing on terminal states.
• Integration: real A2A server (Go if reference: A2A transport alongside MCP on reference agents #69 lands; Python reference otherwise). Prove contextId persists across sends and HITL input-required → resume path works.

Scope

• Sync message/send + tasks/get to start. Streaming added later.
• A2A transport only; MCP gets its own client story.

Non-goals

• Full buyer SDK (discovery, signing abstractions) — this is the transport adapter.
• Backwards compat — net-new package.

Sequence

1. Reference-agent A2A transport (reference: A2A transport alongside MCP on reference agents #69) lands first to give us a Go server to test against.
2. Client implementation.
3. Conformance test running the same scenario over MCP (existing) and A2A (this package).

[bokelley]

Blocked-on: #69 — resurfaces on merge.

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Triaged by Claude Code. Session: https://claude.ai/code/session_01XoWGV6yd5RVzftebEbxZ18

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Generated by Claude Code