refactor(acp): use SDK protocol contracts

[steipete]

What Problem This Solves

The ACP bridge duplicated contracts already guaranteed by @agentclientprotocol/sdk@1.1.0: it inferred the SDK stream message type, rechecked discriminated protocol unions, cast discriminated content blocks, manually listed schema keys, and retained client capability/info state with no production reader.

Why This Change Was Made

Use the SDK's public AnyMessage, SessionUpdate, ContentBlock, and schema definitions directly. Delete dead client metadata state, getters, normalization, and their test. Keep the runtime JSON-object guard because ndJsonStream parses but does not validate arbitrary JSON before this compatibility transform.

User Impact

No behavior change. ACP initialization, protocol-version normalization, session updates, prompt text/images, and schema validation retain their existing behavior. The implementation shrinks by 95 lines.

Evidence

• Five focused ACP suites: 99 tests passed locally on the final rebased head
• git diff --check origin/main...HEAD
• Fresh exact-commit autoreview: clean, no accepted/actionable findings
• Supplemental full changed gate for the same patch before the final unrelated main rebase: https://github.com/openclaw/openclaw/actions/runs/29315688883

[steipete]

Land-ready proof for final rebased head 90ed8b27deec86984c5def34e3b40f2505dfc1f2:

• node scripts/run-vitest.mjs src/acp/client.test.ts src/acp/event-mapper.test.ts src/acp/translator.lifecycle.test.ts src/acp/protocol-schema.test.ts src/acp/server.startup.test.ts: 99 passed locally
• git diff --check origin/main...HEAD: passed
• Fresh exact-commit autoreview: clean, no accepted/actionable findings
• Supplemental: the same patch passed the complete changed gate on the immediately prior rebased head in Blacksmith Testbox: https://github.com/openclaw/openclaw/actions/runs/29315688883

Known gap: hosted CI not awaited, per maintainer instruction. The final head has focused local proof under the current validation policy; its patch is unchanged from the full-gate run.