Add streaming support, request cancellation, and client hardening

[Mako88]

Summary

Adds streaming response support and request cancellation, plus several hardening fixes to the HTTP client, and expands the README.

New features

• MakeStreamRequest returns an ISimpleStreamResponse (IDisposable) exposing the raw, unbuffered response Stream alongside StatusCode / IsSuccessful / Headers. Sends with HttpCompletionOption.ResponseHeadersRead; the caller owns the connection lifetime via disposal.
• CancellationToken added to MakeRequest / MakeRequest<T>. Caller cancellation surfaces as OperationCanceledException; timeouts still surface as TimeoutException.

Hardening fixes

• HttpClient lifetime / thread-safety: guard lazy creation + periodic replacement with a lock (no more duplicate clients/timers under concurrent first-requests), and dispose retired self-created clients after a grace period instead of leaking them. Factory-created clients are left to the factory.
• Request headers: apply via TryAddWithoutValidation and route content-level headers (e.g. Content-Disposition) to the body content, so they no longer throw.
• Response headers: populate via the indexer to avoid throwing on a duplicate key across response/content header collections.

Docs

• Restructured README with a table of contents and a dedicated Streaming section; documented previously-undocumented features (timeouts, params, headers, bodies, additional success codes, serializers, logging). Fixed a stale ISimpleHttpClient reference (the registered interface is ISimpleClient).

Tests

All 74 integration tests pass, including new coverage for the streaming surface, request cancellation, and custom/content-level request headers.

Notes for the maintainer

• Two findings were examined and deliberately left as-is because changing them would break documented/tested behavior or risk correctness: the request.StringBody/ContentType mutation (covered by existing tests) and the double read of the response body (operates on already-buffered content; reworking risks charset/BOM regressions). Happy to revisit either in a separate PR.
• Version bumped to 4.2.0.

🤖 Generated with Claude Code

Solves #3

[Mako88]

Thanks for the thorough PR — the broad direction looks good, and CI is green on the current head.

I found one thing I’d like cleaned up before approval: the streaming cancellation docs/API contract currently imply the CancellationToken passed to MakeStreamRequest cancels later reads from the returned stream. In practice, that token is only used for SendAsync through response headers; after MakeStreamRequest returns a raw Stream, subsequent ReadAsync calls won’t automatically observe the original token. Callers need to pass their token into the stream read operations themselves.

Suggested fix: update the README/XML docs wording to say the token cancels sending the request and waiting for response headers. For reading, callers should use the ReadAsync(..., cancellationToken) overloads where available, or otherwise close/dispose the response/stream to abort the in-flight read.

Everything else I reviewed looks reasonable: the ResponseHeadersRead lifetime is tied to disposing SimpleStreamResponse, headers are now applied after content so content headers can land on HttpContent, duplicate response/content headers are handled via index assignment, and the new provider split makes the client lifetime behavior easier to reason about.