Add expression_valid validator

Implement a new provider::validatefx::expression_valid validator.

Description:

• Validate that a string is a valid HCL expression.

Acceptance criteria:

1. Subtask: Implement expression_valid validator in internal/validators
2. Subtask: Add unit tests for expression_valid validator
3. Subtask: Add expression_valid function to internal/functions package
4. Subtask: Add unit tests for expression_valid function
5. Subtask: Register expression_valid in registry.go
6. Subtask: Add documentation in examples/functions/expression_valid/function.tf
7. Subtask: Update docs table using make docs command
8. Subtask: Add integration test in integration/main.tf