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/migrations/000449_aibridge_session_indexes.down.sql

@@ -0,0 +1,5 @@
+DROP INDEX IF EXISTS idx_aibridge_interceptions_session_id;
+DROP INDEX IF EXISTS idx_aibridge_user_prompts_interception_created;
+DROP INDEX IF EXISTS idx_aibridge_interceptions_sessions_filter;
+
+ALTER TABLE aibridge_interceptions DROP COLUMN IF EXISTS session_id;

coderd/database/migrations/000449_aibridge_session_indexes.up.sql

@@ -0,0 +1,40 @@
+-- A "session" groups related interceptions together. See the COMMENT ON
+-- COLUMN below for the full business-logic description.
+ALTER TABLE aibridge_interceptions
+    ADD COLUMN session_id TEXT NOT NULL
+        GENERATED ALWAYS AS (
+            COALESCE(
+                client_session_id,
+                thread_root_id::text,
+                id::text
+            )
+        ) STORED;
+
+-- Searching and grouping on the resolved session ID will be common.
+CREATE INDEX idx_aibridge_interceptions_session_id
+    ON aibridge_interceptions (session_id)
+    WHERE ended_at IS NOT NULL;
+
+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.';
+
+-- Composite index for the most common filter path used by
+-- ListAIBridgeSessions: initiator_id equality + started_at range,
+-- with ended_at IS NOT NULL as a partial filter.
+CREATE INDEX idx_aibridge_interceptions_sessions_filter
+    ON aibridge_interceptions (initiator_id, started_at DESC, id DESC)
+    WHERE ended_at IS NOT NULL;
+
+-- Supports lateral prompt lookup by interception + recency.
+CREATE INDEX idx_aibridge_user_prompts_interception_created
+    ON aibridge_user_prompts (interception_id, created_at DESC, id DESC);