okhttp4 Phase E: consolidate GzipRequestInterceptor into one shared class

[ahaarrestad]

Summary

Consolidates three separate GzipRequestInterceptor implementations into one shared class, with no observable behavior change. Part of the okhttp4 migration (Phase E — independent of the B1/B2 version bump in #4589; this branch is off master on okhttp 3.12.13).

What was actually there (the premise "3 identical copies" was wrong)

Copy	Visibility	Behavior	Consumers
cgm/nsfollow/GzipRequestInterceptor	public, // TODO make reusable	gzip unless body == null or Content-Encoding present	4 cross-package consumers: SendFeedBack, NightLiteClient, RetrofitService, sharefollow/RetrofitBase
tidepool/GzipRequestInterceptor	package-private	byte-for-byte identical to nsfollow	none — only usage was commented out → dead code
NightscoutUploader.GzipRequestInterceptor	inner static class	different — also gates on !supportsGzip(host+port)	NightscoutUploader

So: 2 identical copies + 1 with a per-server capability gate — not 3 identical copies.

Change

• New shared com.eveningoutpost.dexdrip.utils.framework.GzipRequestInterceptor + a small GzipDecider functional interface.
  • No-arg constructor → always gzip (byte-for-byte equal to the old nsfollow/tidepool behavior); used by the 4 nsfollow consumers.
  • GzipRequestInterceptor(GzipDecider) → per-request gate.
• NightscoutUploader now passes NS_GZIP_DECIDER = req -> supportsGzip(host+port), preserving its per-server gzip gating verbatim (same id construction, same PersistentStore marker ns-end-supports-gzip-, same short-circuit order).
• Migrated the 4 production consumers (+ 4 pre-existing test files) to the shared class.
• Removed the dead tidepool/GzipRequestInterceptor copy and the dead commented-out usage line in TidepoolUploader.

Note: okhttp3.* / okio.* package names are correct and remain so under okhttp 4.x (4.x keeps the okhttp3 package). The deprecated RequestBody.create(MediaType, String) arg order is intentionally left for Phase C (deprecation cleanup); Phase E actually shrinks Phase C's gzip scope from 3 copies to 1.

Testing

• New TDD characterization test pins NightscoutUploader's supportsGzip gate through the shared class (marker true → gzip; absent → passthrough; GET/no-body and already-encoded passthrough).
• New shared-class test covers no-arg (gzip / no-body / already-encoded) + both decider outcomes — real MockWebServer round-trips, gunzip assertions.
• Full ./gradlew test → BUILD SUCCESSFUL, 0 failures.