feat: add pr, repo, pr_title chat search filters (#25569)

Relates to CODAGT-432

Adds three new search filters to the chat list endpoint (`GET
/api/experimental/chats/`):

- `pr:<number>` - exact PR number match
- `repo:<owner/repo>` - substring match against git remote origin or URL
- `pr_title:<text>` - case-insensitive PR title substring match

Includes SQL filter clauses (EXISTS against `chat_diff_statuses`),
parser with validation, handler wiring, unit tests, swagger annotation
update, and a new search syntax documentation page.

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coderd/database/queries/chats.sql

@@ -569,6 +569,39 @@ WHERE
         )
         ELSE true
     END
+    -- Filter by PR number (exact match on chat's diff status).
+    AND CASE
+        WHEN @pr_number::int != 0 THEN EXISTS (
+            SELECT 1
+            FROM chat_diff_statuses cds
+            WHERE cds.chat_id = chats_expanded.id
+                AND cds.pr_number = @pr_number
+        )
+        ELSE true
+    END
+    -- Filter by repository (substring match on remote origin or PR URL).
+    AND CASE
+        WHEN @repo_query::text != '' THEN EXISTS (
+            SELECT 1
+            FROM chat_diff_statuses cds
+            WHERE cds.chat_id = chats_expanded.id
+                AND (
+                    cds.git_remote_origin ILIKE '%' || @repo_query || '%'
+                    OR cds.url ILIKE '%' || @repo_query || '%'
+                )
+        )
+        ELSE true
+    END
+    -- Filter by pull request title (case-insensitive substring).
+    AND CASE
+        WHEN @pr_title_query::text != '' THEN EXISTS (
+            SELECT 1
+            FROM chat_diff_statuses cds
+            WHERE cds.chat_id = chats_expanded.id
+                AND cds.pull_request_title ILIKE '%' || @pr_title_query || '%'
+        )
+        ELSE true
+    END
     -- Paginate over root chats only. Children are fetched
     -- separately via GetChildChatsByParentIDs and embedded under
     -- each parent. Other callers that need the full set should