Add response header configuration

[dennisdoomen]

Closes #113

Summary

Adds the ability to configure response headers on a mocked response, so consumers no longer need to drop down to hand-building an HttpResponseMessage to set headers like Location, ETag, Cache-Control, or a custom Content-Type.

Two additive, fluent methods are added to RequestMockResponseBuilder:

RequestMockResponseBuilder WithHeader(string name, string value);
RequestMockResponseBuilder WithHeader(string name, params string[] values);

Usage:

mock.ForPost().WithPath("/api/users")
    .RespondsWithStatus(HttpStatusCode.Created)
    .WithHeader("Location", "/api/users/123")
    .WithHeader("ETag", "\"v1\"");

How it works

• The mock's Responder is wrapped so that, after it produces the HttpResponseMessage, the configured headers are applied on every invocation.
• Content headers (Content-Type, Content-Length, Content-Encoding, Content-Disposition, Expires, Last-Modified, etc.) are routed to response.Content.Headers; all other headers go to response.Headers.
• Headers are added with TryAddWithoutValidation, so non-standard headers are supported.
• If a content header is configured but the response has no content, an empty ByteArrayContent is created to host it.
• The same header configured twice results in the last value winning (remove-then-add semantics).
• The header name is validated at configuration time (rejects null/empty/white-space) and the values array is defensively copied so later mutation of the caller's array doesn't affect responses.

New public API

• RequestMockResponseBuilder.WithHeader(string name, string value)
• RequestMockResponseBuilder.WithHeader(string name, params string[] values)

The approved API verification files (net472.verified.txt, net8.0.verified.txt) were regenerated via AcceptApiChanges.ps1.

Tests

Added a ResponseHeaders spec class in Mockly.Specs (xUnit, FluentAssertions, AAA) covering: standard header, content-header routing, empty-content creation, multi-value header, header applied on every invocation, chaining, last-value-wins, configuration-time name validation, and array-mutation safety. Full suite is green on net472 and net8.0.

Breaking changes

None. The methods are purely additive. RequestMock.Responder remains a public Func<RequestInfo, HttpResponseMessage>.

Follow-up

This issue also describes companion response-header assertions for the separate dennisdoomen/fluentassertions.mockly repository (WithResponseHeader on CapturedRequestAssertions/ContainedRequestAssertions, for both the v7 and v8 packages). That work is not part of this repository and requires a separate PR in dennisdoomen/fluentassertions.mockly.

[github-actions]

Test Results

  3 files  ± 0    3 suites  ±0   7s ⏱️ -1s
155 tests +10  155 ✅ +10  0 💤 ±0  0 ❌ ±0 
307 runs  +20  307 ✅ +20  0 💤 ±0  0 ❌ ±0 

Results for commit aa4d4f6. ± Comparison against base commit 250a537.

♻️ This comment has been updated with latest results.

[coveralls]

Coverage Report for CI Build 26691388414

Coverage increased (+0.2%) to 84.624%

Details

• Coverage increased (+0.2%) from the base build.
• Patch coverage: 4 uncovered changes across 1 file (44 of 48 lines covered, 91.67%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
Mockly/RequestMockResponseBuilder.cs	48	44	91.67%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	1322
Covered Lines:	1186
Line Coverage:	89.71%
Relevant Branches:	447
Covered Branches:	311
Branch Coverage:	69.57%
Branches in Coverage %:	Yes
Coverage Strength:	283.12 hits per line

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💛 - Coveralls