Add DX helper aliases + boost line coverage to 95.86% (+5.2pp)

[Copilot]

Two related improvements: expressive fluent aliases for every major request feature, and a comprehensive test suite that raises line coverage from 90.66% → 95.86%.

Helper method aliases (src/Httpful/Request.php)

Adds doNotFollowRedirects() / followRedirects() style positive/negative pairs for every major feature area, so callers never need to remember boolean flags:

// Before
$req->followRedirects(false);
$req->enableStrictSsl(false);
$req->withRetry(3, 200, 5000, true, true);

// After — intent is self-documenting
$req->doNotFollowRedirects();
$req->withoutStrictSsl();
$req->withRetryOnAllErrors(3)->withRetryDelay(200)->withRetryMaxTime(5000);

Covers all advertised feature groups:

• Parsing / serialization — withAutoParse(), doNotAutoParse(), withAutoSerialize(), doNotAutoSerialize()
• Auth — withBasicAuth(), withBearerToken(), withClientSideCertificateAuth()
• Retry — withRetryOnConnectionRefused(), withRetryOnAllErrors(), withRetryDelay(), withRetryMaxTime()
• TLS — withStrictSsl() / withoutStrictSsl(), withTlsVersion(), withCaBundle(), withPinnedPublicKey()
• Cookies — withCookieFile(), withCookieJar()
• HTTP version — withHttp2PriorKnowledge(), withHttp3(), withHttp3Only()
• Alt-Svc / HSTS — withAltSvcCache(), withHstsCache()
• Proxy / routing — withNoProxy(), withProxyTunnel(), withResolve(), withConnectTo()
• Transfer metadata — withTransferMetadata(), withoutTransferMetadata()
• Template — withTemplate(), withoutTemplate()
• Download — withDownload(), withoutDownload()
• Parallel — withParallelExecution() (initiates multi-curl)

Test coverage (tests/Httpful/CoverageTopUpTest.php)

New 94-test file targeting previously uncovered branches:

Area	Key paths now covered
Client	delete, get_form, get_xml, post_dom/json/xml, sendRequest, multi download
ClientMulti	add_request() with non-Request PSR-7 adapter branch
Request	client-side cert opt-set paths, _setBody with StreamInterface / duplicate-key, initMulti all four callback if/else branches, _error with PSR-3 logger and callable global handler, wait(false) ErrorException catch
XmlMimeHandler	object serialization via reflection, numeric array keys, boolean scalar
Stream	seek on non-seekable open stream
UriResolver	resolve() with empty base path + non-empty authority; relativize() same-directory edge case
MultiCurlPromise	then() complete & error closure bodies, wait(false) catch via double-add

Fixture files added: tests/static/test.xml, test.csv, test.html, test_form.txt.

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This change is 

[coveralls]

coverage: 95.82% (+5.3%) from 90.483% — copilot/add-missing-helper-methods into master