refactor(request_service): modernize HTTPX transport and purify strategy architecture - #505

What changed

• Drop Faraday remnants and streamline RequestService strategy architecture.
• Enable native HTTPX plugins (follow_redirects, compression, retries) for transport-level resilience, automatic Brotli/Gzip decompression, and redirect loop enforcement.
• Eliminate custom transport-level error wrap layers and unify strategy interface on Strategy base with typed domain errors (RequestTimedOut, BlockedSurfaceDetected, PrivateNetworkDenied, BotasaurusConnectionFailed, BotasaurusServiceError).
• Modernize CompressedBody to handle raw/Botasaurus decompression fallback with unified EncodingError handling.
• Keep strict 0-legacy posture without backward compatibility shims or dual export names.

Why

Faraday was deprecated and replaced with HTTPX, but legacy indirection, intermediate wrapper layers, and bespoke redirect/compression handlers remained in the codebase. This change cuts all residual complexity, leverages HTTPX's native plugin architecture, and aligns RequestService with modern Ruby patterns.

Risk

• Low: Public contract of Html2rss.feed, feed_result, and CLI commands remains intact. All 1,871 test suite examples pass with 98.32% line coverage.

Review map

1. lib/html2rss/request_service/httpx_strategy.rb — Native HTTPX plugins configuration and response building.
2. lib/html2rss/request_service.rb — Clean strategy registry and error classification.
3. lib/html2rss/request_service/strategy.rb — Base strategy interface.
4. lib/html2rss/request_service/compressed_body.rb — Decompression utility.
5. spec/lib/html2rss/request_service_spec.rb — Updated test suite.

Validation

• make ready (1,871 examples, 0 failures; RuboCop clean; YARD lint clean; schema & docs verified).