fix(waterdata): naive datetime filters shifted an hour across the DST boundary

[thodson-usgs]

Problem

_format_api_dates resolved the local timezone once, as a frozen offset from "now":

local_timezone = datetime.now().astimezone().tzinfo   # e.g. timezone(-4h) in summer
... parsed.replace(tzinfo=local_timezone)             # applied to EVERY naive input

A fixed offset can't represent DST, so a naive datetime on a date whose DST status differs from today is shifted by an hour. Verified on an EDT (−04:00) machine:

time="2020-01-01 12:00:00"  ->  2020-01-01T16:00:00Z   # WRONG — January is EST (-05:00), should be 17:00:00Z

This skews the query window for get_continuous / get_field_measurements (time/begin/end) whenever the input date is in the other DST season.

Fix

Interpret a naive input with parsed.astimezone(), which presumes the system local zone and uses the DST rules for that input's own date, then convert to UTC. Inputs that already carry an explicit offset (Z/+HH:MM) are unchanged.

Verification

winter 2020-01-01 12:00 -> 2020-01-01T17:00:00Z   # now correct (EST -05:00)
summer 2020-07-01 12:00 -> 2020-07-01T16:00:00Z   # correct (EDT -04:00)
aware  +00:00            -> 2020-01-01T12:00:00Z   # offset honored
date-only / interval / duration  -> unchanged

16 existing date-formatting tests pass; ruff clean.

No new unit test: the bug only manifests when the test runner's local zone observes DST, so a portable assertion would require mocking the system tz. Verified manually as above.

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