Feat: local mode, and allowing empty/unset API key to be passed for local_mode

[apelsynca]

A bit of refactor (for the tests mostly), also moved function _process_value from client.py to payload_limits.py
And renamed it to apply_payload_limits (underscore was bad cuz async_client.py was also importing it)

In local mode (or if local_mode is 'if_unset_api_key' and the api_key unset) the client on log method call does nothing.

This allows for easy functionale in test environments, dev environments where you dont want to set api_key or just want to stop logtide from working

[Polliog]

Looked at the diff. The apply_payload_limits refactor and the test changes are fine. Three things to fix:

1. (blocking) local_mode is missing in the async client - async_client.py log() only checks self._closed, so with AsyncLogTideClient and local_mode=True logs still get sent. And since local_mode=True allows api_key=None, the async client will attempt to send with a null key. The same guard from the sync client needs to be replicated:

if self._closed or self.options.local_mode is True:
    return
if self.options.local_mode == "if_unset_api_key" and not self.options.api_key:
    return

2. dsn now always overwrites explicit api_url/api_key (models.py __post_init__). Before, explicit values won (if not self.api_url: ...); now dsn always wins and hand-passed values are silently dropped. Intentional?

3. default api_url = production + relaxed validation (models.py). With api_url now defaulting to https://api.logtide.dev, building ClientOptions(api_key="...") without a url or dsn no longer raises and silently points at prod. It used to raise ValueError. A misconfig ends up in production unnoticed.

Minor: dead comment # if self.options and the two TODOs in log(), plus a typo in the error message ("other then" -> "other than").