feat: track client sessions (#22470)

This change adds support for tracking client session IDs in AI Bridge interceptions to enable better session-based auditing.

Depends on https://github.com/coder/aibridge/pull/198  
Fixes https://github.com/coder/internal/issues/1337

The session ID field is optional and not universally supported by all clients.

coderd/database/migrations/000428_aibridge_sessions.down.sql

@@ -0,0 +1,4 @@
+DROP INDEX IF EXISTS idx_aibridge_interceptions_client_session_id;
+
+ALTER TABLE aibridge_interceptions
+DROP COLUMN client_session_id;

coderd/database/migrations/000428_aibridge_sessions.up.sql

@@ -0,0 +1,7 @@
+ALTER TABLE aibridge_interceptions
+ADD COLUMN client_session_id VARCHAR(256) NULL;
+
+COMMENT ON COLUMN aibridge_interceptions.client_session_id IS 'The session ID supplied by the client (optional and not universally supported).';
+
+CREATE INDEX idx_aibridge_interceptions_client_session_id ON aibridge_interceptions (client_session_id)
+WHERE client_session_id IS NOT NULL;