Refactor `SearchInputDecoration` class added maintainHintHeight

[mrjake34]

This commit refactors the SearchInputDecoration class in input_decoration.dart. It adds a new property maintainHintHeight and updates the constructor and methods accordingly. This change improves the flexibility and customization options for the SearchInputDecoration class.

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Pre-launch Checklist

• [ ] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [ ] I listed at least one issue that this PR fixes in the description above.
• [ ] I updated/added relevant documentation (doc comments with ///).
• [ ] I added new tests to check the change I am making, or this PR is [test-exempt].
• [ ] All existing and new tests are passing using flutter test

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