feat(http): add RetryStrategy callback and context-driven configuration

Introduce RetryStrategy, a callback-based retry mechanism that supersedes the legacy exponential-backoff RetryConfig loop. The strategy owns the attempt cap and retry decision for each HTTP attempt, supporting custom logic like Retry-After header parsing via the new RetryOnStatus helper.

Add WithContext and WithLogger methods to configure clients from context objects, enabling per-feature HTTP tracing and HAR collection. TraceToStdout now deduplicates multiple calls by merging configs into a single installed middleware instead of stacking.

Add CommonsHTTPContext interface for context objects to provide logger, trace config, and HAR collector. Implement metadataHARMiddleware for low-cost HAR capture (headers/timing only, no bodies). Add WriteHARFile to serialize HAR entries to disk.

Add traceConfigForLevel to derive trace config from logger verbosity levels (Trace1 for headers, Trace2 for bodies), making -vvv/-vvvv flags automatically enable tracing.

Breaking change: Client.traceMW and related internals are now exposed for deduplication logic; existing code using Trace() or TraceToStdout() continues to work but may see different middleware stacking behavior.

Refs: HAR collection, context-driven configuration, improved retry control

Author: moshloop

.gitignore

@@ -23,3 +23,4 @@ cmd/hx/fixtures/hx
 .ginkgo/
 .gavel/
 .tmp/
+hack/