Add httptest.ResponseRecorder assertions

[nfisher]

Motivation

• Provide a convenient wrapper around *httptest.ResponseRecorder so tests can assert recorded responses using the existing http.Response assertion helpers.

Description

• Add Recorder(rec *httptest.ResponseRecorder) *RecorderAssert and the RecorderAssert type in hammy/httpassert/recorder.go with a pointer to the recorder.
• Implement Status, StatusInRange, Header, HeaderContains, BodyEqual, BodyContains, and BodyMatchesRegexp on RecorderAssert by converting the recorder via Result() and delegating to the existing http.Response assertion functions.
• Return clear failure messages for nil recorders and invalid input (e.g., invalid status range or invalid regexp patterns).
• Add unit tests in hammy/httpassert/recorder_test.go that cover successful and failing assertions, nil recorder handling, invalid ranges, regexp failures, and body-read restoration behavior.

Testing

• Ran go test ./... and all packages succeeded.

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Codex Task