feat: session list API (#23202)

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_Disclaimer:_ _initially_ _produced_ _by_ _Claude_ _Opus_ _4\.6,_ _heavily_ _modified_ _and_ _reviewed_ _by_ _me._

Closes https://github.com/coder/internal/issues/1360

Adds a new `/api/v2/aibridge/sessions` API which returns "sessions".

Sessions, as defined in the [RFC](https://www.notion.so/coderhq/AI-Bridge-Sessions-Threads-2ccd579be59280f28021d3baf7472fbe?source=copy_link), are a set of interceptions logically grouped by a session key issued by the client.  
The API design for this endpoint was done in [this doc](https://github.com/coder/internal/issues/1360).

If the client has not provided a session ID, we will revert to the thread root ID, and if that's not present we use the interception's own ID (i.e. a session of a single interception - which is effectively what we show currently in our `/api/v2/aibridge/interceptions` API).

The SQL query looks gnarly but it's relatively simple, and seems to perform well (~200ms) even when I import dogfood's `aibridge_*` tables into my workspace. If we need to improve performance on this later we can investigate materialized views, perhaps, but for now I don't think it's warranted.

---

_The PR looks large but it's got a lot of generated code; the actual changes aren't huge._

coderd/database/dump.sql

@@ -1099,7 +1099,8 @@ CREATE TABLE aibridge_interceptions (
     client character varying(64) DEFAULT 'Unknown'::character varying,
     thread_parent_id uuid,
     thread_root_id uuid,
-    client_session_id character varying(256)
+    client_session_id character varying(256),
+    session_id text GENERATED ALWAYS AS (COALESCE(client_session_id, ((thread_root_id)::text)::character varying, ((id)::text)::character varying)) STORED NOT NULL
 );
 
 COMMENT ON TABLE aibridge_interceptions IS 'Audit log of requests intercepted by AI Bridge';
@@ -1112,6 +1113,8 @@ COMMENT ON COLUMN aibridge_interceptions.thread_root_id IS 'The root interceptio
 
 COMMENT ON COLUMN aibridge_interceptions.client_session_id IS 'The session ID supplied by the client (optional and not universally supported).';
 
+COMMENT ON COLUMN aibridge_interceptions.session_id IS 'Groups related interceptions into a logical session. Determined by a priority chain: (1) client_session_id — an explicit session identifier supplied by the calling client (e.g. Claude Code); (2) thread_root_id — the root of an agentic thread detected by Bridge through tool-call correlation, used when the client does not supply its own session ID; (3) id — the interception''s own ID, used as a last resort so every interception belongs to exactly one session even if it is standalone. This is a generated column stored on disk so it can be indexed and joined without recomputing the COALESCE on every query.';
+
 CREATE TABLE aibridge_model_thoughts (
     interception_id uuid NOT NULL,
     content text NOT NULL,
@@ -3654,6 +3657,10 @@ CREATE INDEX idx_aibridge_interceptions_model ON aibridge_interceptions USING bt
 
 CREATE INDEX idx_aibridge_interceptions_provider ON aibridge_interceptions USING btree (provider);
 
+CREATE INDEX idx_aibridge_interceptions_session_id ON aibridge_interceptions USING btree (session_id) WHERE (ended_at IS NOT NULL);
+
+CREATE INDEX idx_aibridge_interceptions_sessions_filter ON aibridge_interceptions USING btree (initiator_id, started_at DESC, id DESC) WHERE (ended_at IS NOT NULL);
+
 CREATE INDEX idx_aibridge_interceptions_started_id_desc ON aibridge_interceptions USING btree (started_at DESC, id DESC);
 
 CREATE INDEX idx_aibridge_interceptions_thread_parent_id ON aibridge_interceptions USING btree (thread_parent_id);
@@ -3672,6 +3679,8 @@ CREATE INDEX idx_aibridge_tool_usages_provider_tool_call_id ON aibridge_tool_usa
 
 CREATE INDEX idx_aibridge_tool_usagesprovider_response_id ON aibridge_tool_usages USING btree (provider_response_id);
 
+CREATE INDEX idx_aibridge_user_prompts_interception_created ON aibridge_user_prompts USING btree (interception_id, created_at DESC, id DESC);
+
 CREATE INDEX idx_aibridge_user_prompts_interception_id ON aibridge_user_prompts USING btree (interception_id);
 
 CREATE INDEX idx_aibridge_user_prompts_provider_response_id ON aibridge_user_prompts USING btree (provider_response_id);