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.

aibridge/intercept/apidump/apidump_internal_test.go

@@ -1,4 +1,4 @@
-package apidump //nolint:testpackage // tests unexported internals
+package apidump
 
 import (
 	"bytes"

aibridge/intercept/apidump/headers_internal_test.go

@@ -1,4 +1,4 @@
-package apidump //nolint:testpackage // tests unexported internals
+package apidump
 
 import (
 	"bytes"

aibridge/intercept/apidump/streaming_internal_test.go

@@ -1,4 +1,4 @@
-package apidump //nolint:testpackage // shares test helpers with apidump_test.go
+package apidump
 
 import (
 	"bytes"

aibridge/intercept/chatcompletions/base_internal_test.go

@@ -1,4 +1,4 @@
-package chatcompletions //nolint:testpackage // tests unexported internals
+package chatcompletions
 
 import (
 	"context"

aibridge/intercept/chatcompletions/blocking_internal_test.go

@@ -1,4 +1,4 @@
-package chatcompletions //nolint:testpackage // tests unexported internals
+package chatcompletions
 
 import (
 	"context"

aibridge/intercept/chatcompletions/paramswrap_internal_test.go

@@ -1,4 +1,4 @@
-package chatcompletions //nolint:testpackage // tests unexported internals
+package chatcompletions
 
 import (
 	"fmt"

aibridge/intercept/chatcompletions/streaming_internal_test.go

@@ -1,4 +1,4 @@
-package chatcompletions //nolint:testpackage // tests unexported internals
+package chatcompletions
 
 import (
 	"io"

aibridge/intercept/messages/base_internal_test.go

@@ -1,4 +1,4 @@
-package messages //nolint:testpackage // tests unexported internals
+package messages
 
 import (
 	"context"

aibridge/intercept/messages/blocking_internal_test.go

@@ -1,4 +1,4 @@
-package messages //nolint:testpackage // tests unexported internals
+package messages
 
 import (
 	"io"

aibridge/intercept/messages/reqpayload_internal_test.go

@@ -1,4 +1,4 @@
-package messages //nolint:testpackage // tests unexported internals
+package messages
 
 import (
 	"testing"

aibridge/intercept/messages/streaming_internal_test.go

@@ -1,4 +1,4 @@
-package messages //nolint:testpackage // tests unexported internals
+package messages
 
 import (
 	"context"

aibridge/intercept/responses/base_internal_test.go

@@ -1,4 +1,4 @@
-package responses //nolint:testpackage // tests unexported internals
+package responses
 
 import (
 	"context"

aibridge/intercept/responses/blocking_internal_test.go

@@ -1,4 +1,4 @@
-package responses //nolint:testpackage // tests unexported internals
+package responses
 
 import (
 	"context"

aibridge/intercept/responses/reqpayload_internal_test.go

@@ -1,4 +1,4 @@
-package responses //nolint:testpackage // tests unexported internals
+package responses
 
 import (
 	"encoding/json"

aibridge/intercept/responses/streaming_internal_test.go

@@ -1,4 +1,4 @@
-package responses //nolint:testpackage // tests unexported internals
+package responses
 
 import (
 	"io"

aibridge/internal/integrationtest/apidump_internal_test.go

@@ -1,4 +1,4 @@
-package integrationtest //nolint:testpackage // tests unexported internals
+package integrationtest
 
 import (
 	"bufio"

aibridge/internal/integrationtest/bridge_internal_test.go

@@ -1,4 +1,4 @@
-package integrationtest //nolint:testpackage // tests unexported internals
+package integrationtest
 
 import (
 	"bytes"

aibridge/internal/integrationtest/circuit_breaker_internal_test.go

@@ -1,4 +1,4 @@
-package integrationtest //nolint:testpackage // tests unexported internals
+package integrationtest
 
 import (
 	"fmt"

aibridge/internal/integrationtest/keypool_failover_internal_test.go

@@ -1,4 +1,4 @@
-package integrationtest //nolint:testpackage // tests unexported internals
+package integrationtest
 
 import (
 	"fmt"

aibridge/internal/integrationtest/metrics_internal_test.go

@@ -1,4 +1,4 @@
-package integrationtest //nolint:testpackage // tests unexported internals
+package integrationtest
 
 import (
 	"bytes"

aibridge/internal/integrationtest/responses_internal_test.go

@@ -1,4 +1,4 @@
-package integrationtest //nolint:testpackage // tests unexported internals
+package integrationtest
 
 import (
 	"context"

aibridge/internal/integrationtest/trace_internal_test.go

@@ -1,4 +1,4 @@
-package integrationtest //nolint:testpackage // tests unexported internals
+package integrationtest
 
 import (
 	"context"

aibridge/passthrough_internal_test.go

@@ -1,4 +1,4 @@
-package aibridge //nolint:testpackage // tests unexported newPassthroughRouter
+package aibridge
 
 import (
 	"crypto/tls"

aibridge/provider/anthropic_internal_test.go

@@ -1,4 +1,4 @@
-package provider //nolint:testpackage // tests unexported internals
+package provider
 
 import (
 	"bytes"

aibridge/provider/copilot_internal_test.go

@@ -1,4 +1,4 @@
-package provider //nolint:testpackage // tests unexported internals
+package provider
 
 import (
 	"bytes"

aibridge/provider/openai_internal_test.go

@@ -1,4 +1,4 @@
-package provider //nolint:testpackage // tests unexported internals
+package provider
 
 import (
 	"bytes"