feat: Add full text search over chat messages (#27126)

Closes CODAGT-721
Closes CODAGT-722
Closes CODAGT-723
Closes CODAGT-724
Closes CODAGT-725

This PR adds the database and API pieces necessary to support full-text
chat message search.

- Adds required chat schema for full-text search
- Adds dbpurge job to populate search_tsv in the background
- Adds `search` parameter to GetChats query
- Adds `search` filter to `searchquery.Chats`
- Wires chat search filter into chats API

> Implemented by Coder Agents, reviewed and tested by a human.

coderd/searchquery/search.go

@@ -519,6 +519,8 @@ func Tasks(ctx context.Context, db database.Store, query string, actorID uuid.UU
 //     ownership scope; created_by_me returns only chats the caller owns,
 //     shared_with_me returns only chats shared with the caller, all returns
 //     both)
+//   - search: full-text search over chat content; mutually exclusive
+//     with title, pr_title, and pr
 func Chats(query string) (database.GetChatsParams, []codersdk.ValidationError) {
 	filter := database.GetChatsParams{
 		// Default to hiding archived chats and chats not owned by the caller.
@@ -606,6 +608,30 @@ func Chats(query string) (database.GetChatsParams, []codersdk.ValidationError) {
 		}
 	}
 
+	if values.Has("search") {
+		parser.RequiredNotEmpty("search")
+		if search := parser.String(values, "", "search"); search != "" {
+			var conflicts []string
+			if filter.TitleQuery != "" {
+				conflicts = append(conflicts, `"title"`)
+			}
+			if filter.PrTitleQuery != "" {
+				conflicts = append(conflicts, `"pr_title"`)
+			}
+			if filter.PrNumber != 0 {
+				conflicts = append(conflicts, `"pr"`)
+			}
+			if len(conflicts) > 0 {
+				parser.Errors = append(parser.Errors, codersdk.ValidationError{
+					Field:  "search",
+					Detail: fmt.Sprintf(`"search" cannot be combined with %s`, strings.Join(conflicts, ", ")),
+				})
+			} else {
+				filter.Search = search
+			}
+		}
+	}
+
 	parser.ErrorExcessParams(values)
 	return filter, parser.Errors
 }

coderd/searchquery/search_test.go

@@ -1238,6 +1238,8 @@ func TestSearchChats(t *testing.T) {
 		Query                 string
 		Expected              database.GetChatsParams
 		ExpectedErrorContains string
+		// When non-zero, asserts the exact number of validation errors.
+		ExpectedErrorCount int
 	}{
 		{
 			Name:  "Empty",
@@ -1597,6 +1599,90 @@ func TestSearchChats(t *testing.T) {
 			Query:                 "some random words",
 			ExpectedErrorContains: `unsupported search term: "some random words"`,
 		},
+		{
+			Name:  "Search",
+			Query: "search:foo",
+			Expected: database.GetChatsParams{
+				Archived:  sql.NullBool{Bool: false, Valid: true},
+				OwnedOnly: true,
+				Search:    "foo",
+			},
+		},
+		{
+			Name:  "SearchQuoted",
+			Query: `search:"foo bar"`,
+			Expected: database.GetChatsParams{
+				Archived:  sql.NullBool{Bool: false, Valid: true},
+				OwnedOnly: true,
+				Search:    "foo bar",
+			},
+		},
+		{
+			Name:  "SearchWithStructuralFilters",
+			Query: `repo:coder/coder archived:true search:"foo bar"`,
+			Expected: database.GetChatsParams{
+				Archived:  sql.NullBool{Bool: true, Valid: true},
+				OwnedOnly: true,
+				RepoQuery: "coder/coder",
+				Search:    "foo bar",
+			},
+		},
+		{
+			Name:  "SearchWithAllStructuralFilters",
+			Query: `search:foo archived:true repo:coder/coder diff_url:"https://github.com/coder/coder/pull/1" has_unread:true pr_status:open source:created_by_me`,
+			Expected: database.GetChatsParams{
+				Archived:            sql.NullBool{Bool: true, Valid: true},
+				OwnedOnly:           true,
+				RepoQuery:           "coder/coder",
+				DiffURL:             sql.NullString{String: "https://github.com/coder/coder/pull/1", Valid: true},
+				HasUnread:           sql.NullBool{Bool: true, Valid: true},
+				PullRequestStatuses: []string{"open"},
+				Search:              "foo",
+			},
+		},
+		{
+			Name:                  "SearchRepeated",
+			Query:                 "search:foo search:bar",
+			ExpectedErrorContains: `search: Query param "search" provided more than once`,
+			ExpectedErrorCount:    1,
+		},
+		{
+			Name:                  "SearchConflictsWithTitle",
+			Query:                 "search:foo title:bar",
+			ExpectedErrorContains: `search: "search" cannot be combined with "title"`,
+		},
+		{
+			Name:                  "SearchConflictsWithPrTitle",
+			Query:                 "search:foo pr_title:bar",
+			ExpectedErrorContains: `search: "search" cannot be combined with "pr_title"`,
+		},
+		{
+			Name:                  "SearchConflictsWithPr",
+			Query:                 "search:foo pr:12",
+			ExpectedErrorContains: `search: "search" cannot be combined with "pr"`,
+		},
+		{
+			Name:                  "SearchConflictsOrderIndependent",
+			Query:                 "title:bar search:foo",
+			ExpectedErrorContains: `search: "search" cannot be combined with "title"`,
+		},
+		{
+			Name:                  "SearchConflictsWithMultiple",
+			Query:                 "search:foo title:bar pr:12",
+			ExpectedErrorContains: `search: "search" cannot be combined with "title", "pr"`,
+		},
+		{
+			// The tokenizer rejects trailing colons before search validation runs.
+			Name:                  "SearchBareKey",
+			Query:                 "search:",
+			ExpectedErrorContains: "cannot start or end with ':'",
+		},
+		{
+			Name:                  "SearchEmptyQuoted",
+			Query:                 `search:""`,
+			ExpectedErrorContains: `search: Query param "search" is required and cannot be empty`,
+			ExpectedErrorCount:    1,
+		},
 	}
 
 	for _, c := range testCases {
@@ -1606,6 +1692,9 @@ func TestSearchChats(t *testing.T) {
 			values, errs := searchquery.Chats(c.Query)
 			if c.ExpectedErrorContains != "" {
 				require.True(t, len(errs) > 0, "expect some errors")
+				if c.ExpectedErrorCount > 0 {
+					require.Len(t, errs, c.ExpectedErrorCount, "expected exact error count")
+				}
 				var s strings.Builder
 				for _, err := range errs {
 					_, _ = s.WriteString(fmt.Sprintf("%s: %s\n", err.Field, err.Detail))