feat: decompose bridge server responsibilities

[drown0315]

Summary

Decomposes AskUiBridgeServer into focused bridge modules for Chat ingress, Device WebSocket lifecycle, session events, and session creation, then thins the remaining HTTP adapter.

What Changed

• Extracted Chat ingress parsing and Selection Comment validation into ChatIngress with focused tests.
• Extracted Device WebSocket lifecycle, stream sink behavior, debug fixture handling, control validation, and cleanup into DeviceWebSocketSession.
• Extracted Bridge Session SSE orchestration into BridgeSessionEventStream.
• Extracted Bridge Session creation and Target Device startup validation into BridgeSessionCreator.
• Consolidated repeated JSON object decoding and session-not-found response mapping in AskUiBridgeServer.
• Added and completed issues/0.0.3-bridge-server-decomposition.md progress tracking.

Why

This implements the 0.0.3 bridge server decomposition plan, keeping the HTTP server as the transport adapter while moving domain workflows behind focused, tested module boundaries.

Test Plan

• dart test from apps/bridge

Related Issues

Closes #19
Closes #20
Closes #21
Closes #22
Closes #23