FIX Stop Copilot and A2A targets from silently losing conversation history

[Utkarsh Bahuguna (u7k4rs6)]

Fixes #3077.

Three parts, one commit each:

• Copilot: if a conversation already has turns (besides a system prompt) but no live session, raise instead of opening a new session and sending only the last turn. Same idea as A2A's existing "no upstream context" error. A system prompt alone still starts a session normally.
• A2A: new conversations start with a contextId we generate, so turn 2 continues the same context even when the agent's reply has no contextId or turn 1 errored. If the agent returns its own contextId we switch to it, like before.
• Repeat-send: submit_async now rejects count > 1 when the target lacks editable history and the conversation already has turns, so it fails up front instead of answering each copy blind. Repeat-send on a new conversation still works.

Checked against the real flows (mocked Copilot SDK, real MessageSendService, in-process a2a-sdk server):

• main: two of three repeat-send copies got answered by sessions that only saw the last message; A2A turn 2 raised on protocol 0.3 and 1.0.
• this branch: repeat-send is rejected with a clear error, Copilot raises instead of answering blind, A2A turn 2 works on both protocol versions (the server even adopts our context id).

Tests: Copilot restart case plus system-prompt-only case, A2A own-context and agent-context cases, and the repeat-send reject and new-conversation cases. The new ones fail on main. tests/unit/prompt_target, backend and prompt_normalizer: 3939 passed (4 MCP/tool test files didn't collect locally because httpx2 isn't installed here; they don't touch this code).