Fix Whisper encoder input diagnostic (#32037)
This pull request introduces a small refactor to the Whisper encoder
subgraph validation logic and adds a new unit test to improve code
clarity and test coverage. The main changes are the extraction of input
name validation into a dedicated function and the addition of a test to
check for invalid input names.
**Refactoring and validation improvements:**
* Extracted the input name validation logic from
`WhisperEncoderSubgraph::Validate` into a new helper function
`ValidateWhisperEncoderInputNames` in `subgraph_whisper_encoder.cc` and
declared it in the header file `subgraph_whisper_encoder.h` for improved
code reuse and readability.
[[1]](diffhunk://#diff-1c7fc3b879a5909572c7a602415544b6b8a37c15efdec3b1b417bdb3dc47a491R17-R24)
[[2]](diffhunk://#diff-ca786f63a9d798e56cbc0905436e0c7c675e30d2a88144f77721ecbdff1d329cR13-R14)
* Updated `WhisperEncoderSubgraph::Validate` to use the new
`ValidateWhisperEncoderInputNames` function, simplifying the validation
code and fixing an error in the previous input name check for the
decoder input.
**Testing improvements:**
* Added a new unit test
`WhisperEncoderSubgraphTest.ReportsInvalidSecondInputName` in
`beam_search_test.cc` to verify that the validation function correctly
reports an error when the decoder input name is incorrect. This
increases test coverage for the new validation logic.
[[1]](diffhunk://#diff-782b3e352d8957cd1226d1b87f2127c58efd5ba7a6472528afbdd6baa8232198R6)
[[2]](diffhunk://#diff-782b3e352d8957cd1226d1b87f2127c58efd5ba7a6472528afbdd6baa8232198R16)
[[3]](diffhunk://#diff-782b3e352d8957cd1226d1b87f2127c58efd5ba7a6472528afbdd6baa8232198R27-R36)
