fix: cancellation, timeout classification, and error-handling correctness

[OmarAlJarrah]

Follow-up correctness fixes on top of the developer-experience work in #208, plus a
small internal consolidation and one file-cleanup. No public API changes.

Correctness fixes

• Cancellation ordering in InterruptibleFuture.cancel. The interrupting flag is now
  raised before the worker reference is cleared. The old order left a window where a
  worker could finish its task, skip the unbind spin, return to its pool, and only then
  receive an interrupt aimed at the cancelled call — delivering it to an unrelated task.
• Read-timeout misclassification in the JDK transport. SocketTimeoutException
  extends InterruptedIOException, so the sync path was setting the caller's interrupt
  flag on a plain read timeout and the async mapper returned it un-normalized. Both now
  surface it as a retryable NetworkException, matching the OkHttp transport and the
  retry steps' interrupt carve-out.
• JacksonSerde.from() on an ObjectMapper subclass with unsupported copy().
  Construction no longer fails; it falls back to the supplied mapper.
• OperationRequestAssembler error context. It now catches IllegalArgumentException
  (what url(String) throws) so a base URL that resolves to a malformed URL keeps its
  enriched base-URL/operation context instead of the builder's generic message.
• RequestOptions.maxRetries negative override. Rejected at build time rather than
  being silently reinterpreted downstream as "use the configured default", mirroring
  timeout()'s validation. 0 still disables retries.
• throwOnHttpError error-body buffering. Buffers up to the shared 1 MiB bound
  instead of 4 KiB, so a large error payload is not truncated mid-value only on the
  recovery path.

Internal consolidation

Removes copy-pasted logic that had drifted or could drift apart:

• Single-sources the idempotent HTTP method set as Method.IDEMPOTENT_METHODS;
  RetrySettings.DEFAULT_RETRYABLE_METHODS and RetryPolicySupport derive from it.
• Hoists the interrupt/timeout retry-classification predicate into a shared
  RetryPolicySupport.isNonRetryableInterrupt, so the sync and async retry steps cannot
  classify cancellation differently.
• Extracts URL-valued-header redaction into a single HeaderValueRedactor used by both
  the sync and async instrumentation steps.
• Extracts OkHttp Content-Type resolution into resolveOkHttpContentType, shared by
  the two request-body adapters.
• Drops a now-dead negative guard in RetryPolicySupport.effectiveMaxRetries (the
  builder now rejects a negative per-call maxRetries).

Cleanup

Removes .superpowers/sdd/progress.md, a local planning note that should never have been
tracked. It reached main through #208's squash merge, so it is deleted here.

Verification

Full gated build passes locally (./gradlew build): tests, ktlint, detekt, apiCheck,
and the Kover coverage floor. Regression tests for the behavioral fixes were added under
#208 and remain green.