fix: handle missing id in TimeSeriesID for broken subscription references

[evertoncolling]

Description

When a time series external_id is changed or the time series is deleted, the /timeseries/subscriptions/members API returns members without an id field. The SDK was expecting id to always be present, causing a KeyError.

This PR:

• Makes TimeSeriesID.id optional to align with the API contract
• Adds is_resolved property to check if a reference points to an existing time series

Fixes #2418

Checklist:

• Tests added/updated.
• Documentation updated. Documentation is generated from docstrings - these must be updated according to your change.
  If a new method has been added it should be referenced in cognite.rst in order to generate docs based on its docstring.
• The PR title follows the Conventional Commit spec.

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[sighol]

LGTM!