fix(fcm): encode durations without floating-point rounding errors

[Shubham-Padkonde]

encode_ttl and encode_milliseconds computed the nanoseconds from the float returned by timedelta.total_seconds(), so some durations were sent with rounding errors: an AndroidConfig.ttl of 86400.9 seconds (or timedelta(days=1, microseconds=900000)) was encoded as "86400.899999999s", and 1.123457 as "1.123456999s". The Duration string is now built from the exact days/seconds/microseconds of the timedelta.

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[gemini-code-assist]

Code Review

This pull request refactors duration encoding in firebase_admin/_messaging_encoder.py by introducing an encode_duration helper method. This method uses exact integer fields of datetime.timedelta to avoid floating-point precision issues associated with total_seconds(). Corresponding test cases have been added to tests/test_messaging.py. The feedback suggests using Python's f-string integer formatting {nanos:09d} instead of str(nanos).zfill(9) for cleaner and more idiomatic code.

[gemini-code-assist]

Using Python's f-string integer formatting {nanos:09d} is more idiomatic, cleaner, and slightly more efficient than converting to a string and calling zfill(9).

Suggested change

	if nanos:
	return f'{seconds}.{str(nanos).zfill(9)}s'
	if nanos:
	return f'{seconds}.{nanos:09d}s'