Refactor httpassert API to fluent Response wrapper and update tests/examples

[nfisher]

Motivation

• Provide a more ergonomic, chainable API for HTTP assertions by moving from standalone functions to a Response wrapper with methods.
• Improve nil-safety for receiver usage and streamline example/test usage to the new fluent API.

Description

• Introduce Resp type and Response(resp *http.Response) *Resp constructor and implement methods Status, StatusInRange, Header, HeaderContains, BodyEqual, BodyContains, and BodyMatchesRegexp on it.
• Add a nil-safe helper response() on Resp so method calls on a nil receiver return appropriate assertion failures.
• Update readBody to be reused by the new methods without behavioral changes.
• Update examples in example_test.go and all tests in httpassert_test.go to use the new httpassert.Response(...).<Method> form and rename example functions accordingly.
• Replace the old requireFailure helper with assertions via eye.Spy in tests and add Test_Response_methods_failure_nil_receiver to verify nil-receiver behavior.

Testing

• Ran package tests with go test ./hammy/httpassert and go test ./... and the test suite succeeded.
• Verified the new Test_Response_methods_failure_nil_receiver passes and failure cases are asserted via eye.Spy as expected.
• Examples output updated and validated via the example tests and they succeeded.

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Codex Task