feat(sdk): add streaming extensions and response accumulator

[jezekra1]

Summary

Adds a delta-based streaming protocol using JSON Patch (RFC 6902) with a custom str_ins operation for efficient token-by-token text delivery to the UI.

Server (adk-py)

• MessageAccumulator — 3-level state machine that collects string chunks, parts, and metadata into complete messages, generating incremental JSON Patch operations for the client
• Extended JSON Patch (jsonpatch_ext) — adds a str_ins operation for optimized text streaming without full-value replacement on every token
• StreamingExtensionServer/Client — extension negotiation via agent card capabilities, patch extraction from status update metadata, delta emission, and graceful fallback for non-streaming clients
• Extension base refactoring — activation lifecycle improvements, consolidated DEFAULT_DEMAND_NAME constant (was duplicated in 5 files), improved metadata handling
• RunContext improvements — extracted shared store preparation into _prepare_store_data, added store_sync for synchronous callers
• New yield type — ArtifactChunk for chunked artifact streaming with shared artifact IDs

Client (adk-ts)

• TypeScript streaming extension types, Zod schemas, and STREAMING_EXTENSION_URI constant
• Extension re-exports for clean public API

UI (adk-ui)

• Patch application (streaming.ts) — JSON pointer resolution, support for replace/add/str_ins operations, optimized cloning (skips structuredClone for immutable primitives)
• Client streaming integration — draft accumulation in streamingDraft, patch extraction from TaskStatusUpdateEvent metadata, replace-mode emission
• Bug fix: text duplication on final message — when streaming was active, the final complete message was appended instead of replacing the streaming draft, causing duplicated text (e.g. "Hello! How can I assist you today?Hello! How can I assist you today?"). Now detects active streaming sessions and emits with replace: true
• Performance: batched React state updates — collapsed multiple updateCurrentAgentMessage calls per streaming tick into a single call, reducing re-renders during high-frequency token streaming
• Cleanup — removed dead commented-out proxy header code from adk-client.ts

Wire format

Streaming data is carried in TaskStatusUpdateEvent.metadata[STREAMING_EXTENSION_URI] as a message_update patch list with a message_id. Clients that don't negotiate streaming support receive the final complete message at TASK_STATE_COMPLETED — fully backward compatible.

Test plan

• Unit tests for MessageAccumulator state machine (state transitions, patch output, flush)
• Unit tests for extended JSON Patch str_ins operation
• Unit tests for RunContext store methods
• E2E streaming tests (patch application, message reconstruction, client fallback)
• Updated yield tests for artifact chunking
• mise run check passes (lint, typecheck, license)

[tomkis]

@PetrBulanek can you please review the UI part?