Add fluent HTTP response assertions

[nfisher]

Motivation

• Provide a fluent API for making assertions against *http.Response values to improve ergonomics and chaining while preserving backwards compatibility with existing package-level functions.
• Allow callers to use a Resp wrapper for method-style assertions and ensure graceful behavior for nil receivers.

Description

• Add Response(resp *http.Response) *Resp and the Resp wrapper type with an actual *http.Response field to hammy/httpassert/httpassert.go.
• Implement fluent methods Status, StatusInRange, Header, HeaderContains, BodyEqual, BodyContains, and BodyMatchesRegexp on *Resp and delegate the existing package-level functions to these methods for compatibility.
• Add a response() helper on *Resp to handle nil-receiver cases safely, and reuse the existing readBody helper to preserve body restoration behavior.
• Add tests in hammy/httpassert/httpassert_test.go that cover fluent success scenarios and nil-receiver handling for *Resp.

Testing

• Ran go test ./hammy/httpassert and the package tests passed successfully.
• Ran timeout 60s go test ./... and the full test suite completed successfully (all automated tests passed).

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Codex Task