fix(py): greeting reflects the session that registered its table

[cpsievert]

Stacked on #303

Depends on #303 (don't clean up a table replaced via server(data_source=)) — review that first. This PR's diff is just the greeting-race fix on top of it.

The problem

When no explicit greeting is provided, one is generated by asking the LLM to describe the registered table(s). That generation is asynchronous — it happens after .server() has already returned, whenever the chat UI actually needs to show a greeting.

At that later point, greeting generation reads two things live: the list of table names to describe (greeter.tables) and the QueryChat instance's data sources — both shared, mutable state. With per-session registration:

qc = QueryChat(None, table_name="orders")

def server(input, output, session):
    conn = get_per_user_connection(session)
    qc.server(data_source=conn)

• Session A registers its table and returns from .server(). Its greeting hasn't been generated yet — that happens later, once the chat UI mounts.
• Session B registers its table in the meantime, changing what greeter.tables/the data sources describe.
• Session A's greeting is then generated — and describes session B's table, not session A's.

The fix

.server() now captures a snapshot of greeter.tables synchronously, at call time (alongside the data-sources snapshot it already captured for other reasons), and passes both through to a new private QueryChatGreeter._generate_async_snapshot() used only by the Shiny module's greeting callback. The async greeting generation reads that snapshot instead of live state.

This intentionally isn't exposed on the public generate()/generate_async()/build_client() API — it's an internal fix for a race the per-session registration pattern introduces, not a new feature for users to reach for.

Test plan

• New tests on QueryChatGreeter._generate_async_snapshot(): an explicit tables/data_sources snapshot is used instead of live state.
• New test on mod_server(): the greeting callback calls _generate_async_snapshot() with the snapshot it was given, not the live greeter.
• New test on .server(): it passes greeting_tables=list(self.greeter.tables) through to mod_server().
• New tests locking in mixed config-time add_table() + per-session server(data_source=) behavior (parity with fix(r): let $server(data_source=) survive a second session safely #306's R tests): unnamed registration replaces the config-time table without cleaning it up, an explicit table_name adds alongside it, and the registry is shared and cumulative across sessions.