Fixes #9377

In group chats, an empty mention (@bot with no text) starts a 60s
session_waiter to wait for follow-up content. The waiter is registered with
DefaultSessionFilter, whose key is unified_msg_origin (group-level) and does
not include the sender. As a result, the first non-empty message from any
other member during the wait window is intercepted, gets a bot At prepended,
and is re-dispatched — producing a reply to the wrong user / wrong context. Full
analysis, reproduction and captured logs are in #9377.

This PR fixes the empty-mention flow without changing the default session
scope, so third-party plugins that rely on the current group-level default are
unaffected.

Why not simply change DefaultSessionFilter

Changing the default from unified_msg_origin to include the sender would
change the default runtime behavior of session_waiter, which is exported to
plugins through astrbot.api.util. The group-level default has been in place
since v3.5.4 (~15 months, 138 stable releases) and is relied upon by real
plugins that omit session_filter=, for example:

• astrbot_plugin_kimi_datasource_api registers a waiter manually with
  SessionWaiter(DefaultSessionFilter(), event.unified_msg_origin, ...). The
  registration key is the raw UMO while lookups go through
  DefaultSessionFilter.filter(event). If the default changed, the keys would no
  longer match and its login waiter would never trigger.
• astrbot_plugin_decision_roulette and astrbot_plugin_TurtleSoup use the
  default filter to collect input from multiple group members; a per-sender
  default would only accept the initiator.

So the fix is scoped to the one place that actually needs per-sender isolation.

Modifications

• astrbot/core/utils/session_waiter.py: add SenderSessionFilter, a new
  optional SessionFilter keyed by unified_msg_origin + sender_id.
  DefaultSessionFilter is unchanged, and no new names are added to the
  public astrbot.api.util surface. This is purely additive.
  The key uses a length-prefixed encoding (f"{len(umo)}:{umo}:{sender}")
  rather than plain concatenation: both the UMO and the sender id are arbitrary
  platform strings (e.g. Telegram topic-group UMOs contain #), so a plain
  separator would allow constructible key collisions that defeat the isolation.
• astrbot/builtin_stars/astrbot/main.py: the empty-mention waiter now passes
  session_filter=SenderSessionFilter(). When the sender id is unavailable it
  skips waiting entirely, to avoid silently degrading back to group scope.
• tests/test_session_waiter_cross_user.py (new): 7 session-key regression
  tests (module loaded standalone).
• tests/unit/test_empty_mention_sender_scope.py (new): 3 production-wiring
  tests driving the real Main.handle_empty_mention and
  Main.handle_session_control_agent (following the existing
  Main.__new__(Main) pattern in tests/unit/test_group_chat_context_wiring.py).

The other issues noted in #9377 (empty-message_str messages being swallowed
during the wait window; _cleanup / FILTERS behavior with same-key waiters)
are independent of the session key and will be handled in separate PRs.

Screenshots or Test Results

$ ruff format --check <changed files + test files> && ruff check <same>
All checks passed!

$ pytest tests/test_session_waiter_cross_user.py \
         tests/unit/test_empty_mention_sender_scope.py -o asyncio_mode=auto -q
..........                                                               [100%]
10 passed

Session-key coverage (tests/test_session_waiter_cross_user.py):

1. test_default_filter_returns_umo_unchanged — the default filter still returns
   unified_msg_origin (locks the default semantics in place);
2. test_default_manual_umo_registration_still_triggers — reproduces the
   kimi_datasource_api manual-registration pattern and asserts it still
   triggers (guards against regressing plugins that depend on the group default);
3. test_sender_filter_isolates_users_in_group — another member's message is no
   longer intercepted (primary goal);
4. test_sender_filter_accepts_original_sender — the original sender's follow-up
   is still captured;
5. test_sender_filter_key_is_collision_free — UMOs / sender ids containing
   separator characters (#, :) cannot produce colliding keys;
6. test_sender_filter_isolates_users_with_hash_in_umo — isolation holds for
   Telegram topic-group style UMOs that contain #;
7. test_sender_filter_does_not_leak_across_conversations — the same user's
   messages in other groups / DMs are not intercepted.

Production-wiring coverage (tests/unit/test_empty_mention_sender_scope.py,
drives the real handlers rather than a re-implementation):

8. test_other_members_message_passes_through — BOB's message is not stopped,
   not modified, and not re-enqueued while ALICE's waiter is active;
9. test_original_senders_followup_is_redispatched — ALICE's follow-up gets the
   bot At injected, is stopped and re-enqueued, and the waiter is cleaned up;
10. test_empty_sender_id_skips_waiting — with no reliable sender id the
    handler finishes immediately and registers no waiter.

Verified that the wiring tests actually guard the fix: with the main.py
change reverted to master, tests 8 and 10 fail; with the fix in place all 10
pass.

• This is NOT a breaking change.

The default session scope is unchanged; only the built-in empty-mention waiter
opts into per-sender isolation. SenderSessionFilter is an additive core
utility and is not exported through astrbot.api.util.

Checklist

• 😊 If new features are added, they have been discussed with the authors
  via issues/emails. (Bug fix, proposed in [Bug] 群聊中空提及等待器会截获其他成员的消息（session_waiter 未绑定发送者） #9377.)
• 👀 My changes have been well-tested, with verification steps and results
  provided above.
• 🤓 No new dependencies are introduced.
• 😮 My changes do not introduce malicious code.

Summary by Sourcery

Fix empty-mention session handling in group chats by isolating the waiter per sender while preserving existing group-level session semantics.

Bug Fixes:

• Prevent empty-mention waiters in group chats from intercepting follow-up messages from other members and replying in the wrong context.
• Ensure empty-mention handling skips session waiting when no reliable sender ID is available to avoid unintended group-scoped behavior.

Enhancements:

• Introduce an optional SenderSessionFilter that scopes session keys by conversation origin and sender using a collision-safe encoding without changing the default session filter.
• Strengthen session key behavior with explicit guarantees against key collisions and cross-conversation leakage.

Tests:

• Add regression tests that lock in DefaultSessionFilter behavior and validate SenderSessionFilter isolation, collision safety, and cross-conversation separation.
• Add production wiring tests for empty-mention handling to verify correct per-sender redispatch, non-interference with other members, and behavior when sender ID is missing.