fix: capture request date before network request to prevent clock skew/suspension bugs

[paulb777]

Description

Fixes #16573

Previously, AppCheckCore counted a cached token's time-to-live starting from the moment the token response was processed locally ([NSDate date]). Because we don't trust device clock skew, parsing the JWT exp claim directly is undesirable. However, counting from the response processing time introduces a bug where any delay (e.g., app suspension or a stalled thread) between the token request and the response handler executing is silently converted into extra token lifetime.

This PR implements the recommended fix: taking a snapshot of the device timestamp before sending the token exchange request, and using that timestamp instead of generating a new one after the response is received.

Changes

• Updated _GACURLSessionDataResponse to capture and store a requestDate property.
• Updated NSURLSession+GACPromises.m to snapshot [NSDate date] right before invoking dataTaskWithRequest and passing it to the response object.
• Modified appCheckTokenWithAPIResponse: in GACAppCheckAPIService.m and GACAppAttestAPIService.m to pass down response.requestDate to initWithTokenExchangeResponse:requestDate:error:.
• Updated all associated unit tests to instantiate _GACURLSessionDataResponse with the new requestDate initializer correctly.