Use datetime for ConsumerConfig timestamps

[caspervonb]

• Parse ISO 8601 timestamps from the server into datetime objects in from_response
• Serialize back to ISO 8601 with Z suffix in to_request
• Add tests for opt_start_time round-trip and datetime types on created/timestamp fields

[Copilot]

Pull request overview

This PR updates JetStream consumer timestamp configuration fields to use datetime objects (instead of raw numeric values) and adds tests asserting that various timestamps returned by the API are timezone-aware datetime instances.

Changes:

• Change ConsumerConfig.opt_start_time / pause_until to datetime | None, including request serialization and response parsing.
• Add tests for timezone-aware datetime timestamps on StreamInfo (created, timestamp, state.first_ts, state.last_ts).
• Add tests for consumer timestamps (ConsumerInfo.created, ConsumerInfo.timestamp) and opt_start_time round-tripping.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
nats-jetstream/src/nats/jetstream/consumer/init.py	Switch ConsumerConfig timestamp fields to datetime, serialize to RFC3339, parse from API responses.
nats-jetstream/tests/test_stream.py	Add assertions that stream timestamps are timezone-aware datetime values.
nats-jetstream/tests/test_consumer.py	Add coverage for opt_start_time round-trip and timezone-aware consumer info timestamps.

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[Copilot]

ConsumerConfig.from_response() parses opt_start_time/pause_until with datetime.fromisoformat() without normalizing RFC3339 'Z' suffix. Elsewhere in the codebase (e.g., ConsumerInfo/StreamInfo parsing) timestamps first replace 'Z' with '+00:00'; doing the same here avoids ValueError and keeps parsing consistent.

Suggested change

	opt_start_time = datetime.fromisoformat(opt_start_time_str) if opt_start_time_str is not None else None
	pause_until_str = config.pop("pause_until", None)
	pause_until = datetime.fromisoformat(pause_until_str) if pause_until_str is not None else None
	opt_start_time = (
	datetime.fromisoformat(opt_start_time_str.replace("Z", "+00:00"))
	if opt_start_time_str is not None
	else None
	)
	pause_until_str = config.pop("pause_until", None)
	pause_until = (
	datetime.fromisoformat(pause_until_str.replace("Z", "+00:00"))
	if pause_until_str is not None
	else None
	)

[Jarema]

@caspervonb I think we should address this?

[Copilot]

This test hard-codes a specific calendar date and only asserts individual fields (year/month/day/hour), which can miss a lost/changed timezone and makes the test less robust than asserting full round-trip equality. Consider asserting tzinfo is preserved (and/or comparing the resulting datetime to the original within an acceptable precision).