Review fixes for google-http-client-1.19:javaagent

[otelbot]

Automated code review of instrumentation/google-http-client-1.19/javaagent.

gpt-5.3-codex

Status	File	Category	Note
Fixed	build.gradle.kts:11	Build	Added assertInverse.set(true) to muzzle pass block for inverse range validation.
Fixed	build.gradle.kts:20	Build	Moved collectMetadata system property to withType<Test>().configureEach so custom test tasks inherit it.
Fixed	GoogleHttpRequestInstrumentation:41	Javaagent	Removed redundant isMethod() from transform() method matchers.
Fixed	HttpHeaderSetter:11	Style	Replaced enum singleton TextMapSetter with class-based implementation and usage via new HttpHeaderSetter().
Fixed	HttpHeaderSetter:15	Style	Added upstream-contract @Nullable carrier parameter and null guard in TextMapSetter#set.
Fixed	AbstractGoogleHttpClientTest:48	Testing	Changed test base class to package-private visibility.

Totals: Fixed 6, Needs Manual Fix 0

claude-sonnet-4.6

No issues found.