fix: support ACP audio prompt content

[amknight]

Summary

• add ACP audio prompt block validation and display handling
• reject rich prompt content before session/prompt when the agent does not advertise the matching promptCapabilities
• map runtime audio/* attachments into ACP audio content blocks
• persist and replay-display submitted audio prompt content in session records and the replay viewer

Why

ACP requires clients to restrict prompt content according to agentCapabilities.promptCapabilities. acpx previously accepted and sent image/resource prompt blocks without checking the negotiated capability set, and it did not accept ACP audio content blocks even though ACP defines them.

Validation

• pnpm run format:check
• pnpm run typecheck
• pnpm run lint
• pnpm run build
• pnpm run viewer:typecheck
• pnpm run viewer:build
• pnpm run build:test
• node --test dist-test/test/prompt-content.test.js dist-test/test/client.test.js dist-test/test/runtime-manager.test.js dist-test/test/session-conversation-model.test.js dist-test/test/session-persistence.test.js dist-test/test/persisted-key-policy.test.js dist-test/test/queue-messages.test.js dist-test/test/runtime-events.test.js
• node --test --test-name-pattern "structured ACP prompt blocks" dist-test/test/cli.test.js

Known validation note

pnpm run test:coverage currently stops on one unrelated existing CLI routing test on origin/main: CLI resolves unknown raw agent commands after newer global flags. In that run, 704 tests passed and that one test failed before the coverage phase executed.