Add hammy reader assertions

[nfisher]

Motivation

• Add reader-focused assertions to Hammy so tests can assert io.Reader contents (string/bytes) and emptiness using the same style as existing buffer assertions.
• Ensure assertions are practical for real readers by documenting and handling the fact that reading consumes the supplied io.Reader and by reusing a buffered copy for repeated assertions.

Description

• Add Reader(actual io.Reader) *ReaderAssert and a ReaderAssert type in hammy/reader.go with methods EqualToString, ContainsString, EqualToBytes, IsEmpty, and NotEmpty implemented to return AssertionMessage results.
• Implement a readAll helper that reads and caches the reader body on first use, returns clear failure messages for nil readers and read errors, and reuses the cached body for subsequent assertions.
• Document the reader-consumption behavior in the file-level comment so callers know that the supplied reader is consumed unless it implements its own buffering/reset.
• Add hammy/reader_test.go with tests covering success and failure cases for each assertion, nil-reader and read-error handling, and verification that the buffered body is reused after the first read.

Testing

• Ran go test ./... and all tests passed successfully.

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Codex Task