feat!: stateless per-request agents; add history + conversation_id to send_message

Make Pallas truly stateless per the 'Pallas is ephemeral' contract.

BREAKING (behavioural, not API):
  * instance_scope changes from 'shared' to 'request' in pallas.server.
    Each MCP tools/call now acquires a freshly-created fast-agent instance
    via the existing create_instance / dispose_instance factories and
    disposes it immediately after the response.

With 'shared' mode:
  * Every MCP caller saw the same agent.message_history, so different
    Daedalus conversations leaked into each other.
  * Mid-chat context was silently truncated once the model window filled.
  * Restarting the Pallas process wiped all in-flight conversation state,
    even though Daedalus had it persisted in Postgres.

With 'request' mode the Pallas process holds no per-conversation state;
the caller (Daedalus) owns history and reseeds it on every turn.

send_message gains two optional arguments:
  * history: list[{role, content, images?}] in chronological order,
    converted to PromptMessageExtended and seeded onto the fresh
    instance's message_history before agent.send().
  * conversation_id: opaque string, logged for trace correlation only —
    Pallas never interprets or persists it.

Malformed history entries (bad role, missing image data/mime_type, etc.)
are skipped with a warning rather than raising, so a single bad row
cannot wipe a whole conversation.

The {agent}_history MCP prompt is still registered under 'request'
scope for backward compatibility but always returns []; history lives
on the client.

Version bumped to 0.2.0.

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "pallas-mcp"
-version = "0.1.0"
+version = "0.2.0"
 description = "FastAgent MCP Bridge — generic runtime for serving FastAgent agents over StreamableHTTP"
 requires-python = ">=3.13.5"
 dependencies = [