chore: standardize on *_internal_test.go for white-box tests (#25601)

My agent added `//nolint:testpackage` to a test file on one of my PRs.
Again. This PR cleans it up across the entire repo and updates the
in-repo conventions so future agents stop doing it.

The repo already has a precedent for white-box tests that need to touch
unexported symbols: `*_internal_test.go` (145+ existing files). The
`testpackage` linter's default `skip-regexp` exempts that filename
suffix, so the `//nolint:testpackage` directive is unnecessary in every
case where someone reached for it. This PR renames 51 such files to
`*_internal_test.go` via `git mv` so blame and history follow, and
strips the dead directive from 2 files that were already correctly named
(`coderd/oauth2provider/authorize_internal_test.go`,
`coderd/x/chatd/advisor_internal_test.go`).

`.claude/docs/TESTING.md` now documents the rule explicitly under *Test
Package Naming*, which is imported into the root `AGENTS.md` via
`@.claude/docs/TESTING.md`. The rule: prefer `package foo_test`; if you
need internal access, rename the file to `*_internal_test.go` rather
than adding a nolint directive.

coderd/oauth2provider/authorize_internal_test.go

@@ -1,4 +1,3 @@
-//nolint:testpackage // Internal test for unexported hashOAuth2State helper.
 package oauth2provider
 
 import (

coderd/templates_meta_update_internal_test.go

@@ -1,4 +1,4 @@
-package coderd //nolint:testpackage // Tests the unexported resolveTemplateMetaUpdate helper.
+package coderd
 
 import (
 	"reflect"

coderd/x/chatd/advisor_internal_test.go

@@ -1,4 +1,4 @@
-package chatd //nolint:testpackage // Accesses unexported advisor helpers.
+package chatd
 
 import (
 	"context"

coderd/x/chatd/attachments_internal_test.go

@@ -1,4 +1,4 @@
-package chatd //nolint:testpackage
+package chatd
 
 import (
 	"context"

coderd/x/chatd/chatdebug/model_coverage_internal_test.go

@@ -1,4 +1,4 @@
-package chatdebug //nolint:testpackage // Checks unexported normalized structs against fantasy source types.
+package chatdebug
 
 import (
 	"reflect"

coderd/x/chatd/chatdebug/model_normalization_internal_test.go

@@ -1,4 +1,4 @@
-package chatdebug //nolint:testpackage // Uses unexported normalization helpers.
+package chatdebug
 
 import (
 	"context"

coderd/x/chatd/chatdebug/recorder_internal_test.go

@@ -1,4 +1,4 @@
-package chatdebug //nolint:testpackage // Uses unexported recorder helpers.
+package chatdebug
 
 import (
 	"context"

coderd/x/chatd/chatdebug/reuse_step_internal_test.go

@@ -1,4 +1,4 @@
-package chatdebug //nolint:testpackage // Uses unexported recorder helpers.
+package chatdebug
 
 import (
 	"context"

coderd/x/chatd/chatdebug/transport_internal_test.go

@@ -1,4 +1,4 @@
-package chatdebug //nolint:testpackage // Uses unexported recorder helpers.
+package chatdebug
 
 import (
 	"bytes"

coderd/x/chatd/chatloop/chatloop_run_internal_test.go

@@ -1,4 +1,4 @@
-package chatloop //nolint:testpackage // Uses internal symbols.
+package chatloop
 
 import (
 	"context"

coderd/x/chatd/chatloop/compaction_internal_test.go

@@ -1,4 +1,4 @@
-package chatloop //nolint:testpackage // Uses internal symbols.
+package chatloop
 
 import (
 	"context"

coderd/x/chatd/chattool/askuserquestion_internal_test.go

@@ -1,4 +1,4 @@
-package chattool //nolint:testpackage // Uses internal symbols.
+package chattool
 
 import (
 	"testing"

coderd/x/chatd/chattool/createworkspace_internal_test.go

@@ -1,4 +1,4 @@
-package chattool //nolint:testpackage // Uses internal symbols.
+package chattool
 
 import (
 	"context"

coderd/x/chatd/configcache_internal_test.go

@@ -1,4 +1,4 @@
-package chatd //nolint:testpackage // Uses internal cache state.
+package chatd
 
 import (
 	"context"

coderd/x/chatd/dialvalidation_internal_test.go

@@ -1,4 +1,4 @@
-package chatd //nolint:testpackage // Uses internal symbols.
+package chatd
 
 import (
 	"context"

coderd/x/chatd/instruction_internal_test.go

@@ -1,4 +1,4 @@
-package chatd //nolint:testpackage // Uses internal symbols.
+package chatd
 
 import (
 	"encoding/json"

coderd/x/chatd/quickgen_internal_test.go

@@ -1,4 +1,4 @@
-package chatd //nolint:testpackage // Keeps internal helper tests in-package.
+package chatd
 
 import (
 	"context"

coderd/x/chatd/store_chat_attachment_internal_test.go

@@ -1,4 +1,4 @@
-package chatd //nolint:testpackage
+package chatd
 
 import (
 	"context"

coderd/x/chatd/title_override_internal_test.go

@@ -1,4 +1,4 @@
-package chatd //nolint:testpackage // Tests internal title override helpers.
+package chatd
 
 import (
 	"context"

coderd/x/chatd/usagelimit_internal_test.go

@@ -1,4 +1,4 @@
-package chatd //nolint:testpackage // Keeps chatd unit tests in the package.
+package chatd
 
 import (
 	"testing"