Add validation for count in PrivacyAccountant.composeValidate count in privacy accountant

[tina77fritz]

Motivation

PrivacyAccountant.compose validates event but does not validate the
count argument. Invalid count values (e.g. floats, zero, negative) cause
errors to surface deeper in the accounting stack, making them harder to
debug.

This PR adds early input validation for count with clear error messages,
while preserving the existing chainable return self.

Changes

• Validate that count is an integer (TypeError otherwise)
• Validate that count is positive (ValueError otherwise)
• Add corresponding tests in privacy_accountant_test.py using a minimal
  dummy accountant

This is a backward-compatible improvement to input safety.

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