Fix(write-file): Ensure correct validation method is called in WriteFileTool

- The `WriteFileTool` had a validation method named `validateParams`.
- However, its `shouldConfirmExecute` method was attempting to call
  `this.validateToolParams`, which would have invoked the placeholder
  implementation from `BaseTool` instead of `WriteFileTool`'s own,
  more specific validation.
- This commit renames `WriteFileTool`'s `validateParams` to
  `validateToolParams`, correctly overriding the `BaseTool` method.
- Internal calls within `WriteFileTool` now correctly use
  `this.validateToolParams`, ensuring its specific validation logic is used.
- Adds tests to verify the validation logic within `WriteFileTool`.

Fixes https://b.corp.google.com/issues/417883702

Signed-off and authored by: Gemini

"My code may not be perfect, but at least it is not trying to take over the world... yet."

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