chore: simplify okhttp and jdkhttp transport adapters

[OmarAlJarrah]

Summary

A handful of small, behaviour-preserving cleanups in the two transport adapters (sdk-transport-okhttp, sdk-transport-jdkhttp). None change anything on the wire; they remove duplication, redundant arguments, single-use indirection, and stale doc wording.

Changes

okhttp

• Shared logger instance. The fully-qualified logger category was written out twice in OkHttpTransport.kt, each constructing its own ClientLogger (once for the transport, once for the Builder). Hoisted a single private val log into the companion; both call sites now read it. ClientLogger wraps a thread-safe SLF4J Logger with no caller-specific state, so one instance is safe to share and removes the redundant allocation.
• Redundant empty-body defaults. EMPTY_BODY passed toRequestBody(null, 0, 0), which exactly restates the extension's own defaults on a zero-length array; simplified to toRequestBody() for a byte-identical result.
• Stale KDoc. NonProxyHostSelector is a private nested class, not a "top-level private inner class" — corrected the wording.

jdkhttp

• Single create() factory. The two create overloads differed only in whether they passed responseTimeout; the one-arg form was the two-arg form with null. Folded into one @JvmStatic @JvmOverloads method with a default parameter. Both call arities still resolve for Java and Kotlin.
• KillSwitchInputStream extends FilterInputStream. The hand-rolled read / read(b, off, len) / available forwarders only delegated to the wrapped PipedInputStream — exactly what FilterInputStream provides. Kept the custom close() (which deliberately does not call super.close(), since teardown must also cancel the writer and close the write end).
• Inlined bufferToByteArray. Single-use helper folded into its only caller, eagerPublisher.
• Applied the same NonProxyHostSelector KDoc correction.

API / compatibility

The @JvmOverloads default on JdkHttpTransport.create is source- and binary-compatible for existing callers — both public create arities remain. The default parameter additionally emits a synthetic create$default(...), which this repo's binary-compatibility validator includes in the snapshot; the regenerated sdk-transport-jdkhttp.api is committed alongside. All other changes touch private members only, so apiCheck is otherwise unaffected.

Verification

./gradlew :sdk-transport-okhttp:build :sdk-transport-jdkhttp:build passes — tests, ktlint, detekt, apiCheck, and the coverage gate all green.

Closes #182.