Refactor yamlassert to fluent String/Bytes/Reader assertion types and update examples/tests

[nfisher]

Motivation

• Simplify and unify the YAML assertion API by providing typed assertion builders for string, byte slice, and reader inputs to enable fluent method calls.
• Make the YAML assertions more explicit and discoverable via String, Bytes, and Reader entry points instead of many top-level functions.

Description

• Introduced StringAssert, BytesAssert, and ReaderAssert types and constructors String(), Bytes(), and Reader() and converted top-level functions into methods such as EqualTo, EqualToWithOptions, IsValid, Contains, PathExists, PathMissing, PathEqual, ArrayContains, DocumentCount, DocumentEqual, and DocumentContains on those types.
• Reworked EqualReader/EqualBytes/Equal flows to use the new fluent API internally and preserved option handling via Option (e.g. IgnorePaths and UnorderedArraysAt).
• Updated hammy/README.md, the hammy/yamlassert examples (example_test.go), and unit tests (yamlassert_test.go) to use the new API names and method calls, and updated the feature checklist to reflect the nested String/Bytes/Reader assertions.

Testing

• Ran unit tests for the package using go test ./... and updated tests in hammy/yamlassert to use the new API, and all tests passed.
• Updated example tests in hammy/yamlassert/example_test.go to assert behavior via the new builders and verified their output in test runs.

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Codex Task