tools/template-check: add comprehensive unit tests for valid/invalid functions

[vr-ibm]

Description
This is Part 2 of the template-check func-check series. This PR expands the test suite to cover a wide range of valid and invalid template scenarios.
Changes:

• Replaced minimal tests with a table-driven test suite.
• Added coverage for:
  • MMv1 Custom Functions: camelize, underscore, plural, etc.
  • Go Built-ins: len, index, and, or, etc.
  • Template Keywords: if, range, with, block.
  • Edge Cases: Dot access (.Resource.Name), variables ($var), and string literals.
• Added negative tests for common developer mistakes (e.g., using camelCase instead of camelize).
  Note: This PR is based on add-func-check-logic. It should be merged after Part 1.
  Release Note Template for Downstream PRs (will be copied)

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes for commit 317a0f5:

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