Add optional persistent per-worker HTTP clients - #1

Summary

• add an opt-in LinkConnectionMode::PersistentPerWorker mode while preserving per-request clients as the default
• retain at most one ESP-IDF HTTP client per worker, with no cross-worker sharing or concurrent handle use
• reuse clients only for normalized same-origin URLs and replace them for origin changes, idle expiry, request-count limits, poisoned sessions, and shutdown
• scrub caller headers and POST data after every transfer before queue-owned storage is released
• keep existing redirect security behavior and never automatically replay failed requests
• expose request, client lifecycle, eviction, and transport-event diagnostics
• split the template implementation into focused internal headers

Configuration

LinkConfig config;
config.connectionMode = LinkConnectionMode::PersistentPerWorker;
config.persistentIdleTimeoutMs = 5U * 60U * 1000U;
config.persistentMaxRequestsPerHandle = 0;

Both lifetime limits accept zero as unlimited.

Safety properties

• one handle is owned by exactly one worker
• active persistent clients are bounded by maxConcurrentRequests
• scheme, case-insensitive host, and effective port define the reuse origin
• cross-origin redirects replace the retained handle and retain the existing header-stripping policy
• setup, transfer, callback, buffering, or request-scrub failures poison and clean up the retained handle
• POST, PUT, PATCH, and DELETE requests are not automatically retried
• worker shutdown cleans retained handles before runtime storage is released

Validation

• added host tests for defaults, reuse decisions, idle wraparound, default ports, host normalization, IPv6 origins, and invalid ports
• locally compiled and ran the persistent-client host tests with -Wall -Wextra -pedantic
• locally syntax-compiled the ESP32 code path against an API-compatible ESP-IDF harness
• locally passed the embedded source audit for forbidden dynamic STL and exception constructs
• added a dedicated GitHub Actions workflow for the new host tests

Draft validation remaining

• run the normal repository CI across supported ESP32 targets
• perform an on-device long-duration same-origin HTTPS stress test
• compare internal free heap, largest free block, low-water mark, handle creates/reuses/cleanups, and transport reconnects against PerRequest mode