fix(matrix): prevent echo loop by using /whoami user_id for self-message filtering

[sliverp]

Summary

Fixes #757 — Matrix bot gets stuck in an infinite echo loop.

Root Cause

The Matrix adapter's echo guard compared incoming message senders against the config-provided user_id instead of the server-validated identity from /whoami. When these differ (wrong casing, empty config value, different homeserver), the bot fails to recognize its own messages and replies to them indefinitely.

Changes

1. Echo guard: use server-validated user_id (primary fix)

• start() now uses the /whoami-validated user identity for echo filtering instead of self.user_id
• Added a warning log when the config user_id diverges from the server-reported value

2. Fix txn_id reuse across message chunks

• Moved uuid::Uuid::new_v4() generation inside the chunk loop in api_send_message()
• Previously, a single txn_id was generated once and reused for all chunks — Matrix's PUT idempotency silently dropped all chunks after the first

3. Tests

• test_matrix_adapter_empty_user_id: empty config user_id doesn't panic on construction
• test_sender_echo_filter_semantics: case-sensitive exact match verification for the echo guard
• test_txn_id_uniqueness: UUID uniqueness across message chunks

Testing

All existing tests pass. New tests cover the specific failure modes that caused #757.