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!

[haakonvt]

🦄 Thanks for making a fix!

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.84%. Comparing base (7c6f363) to head (53d62b9).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...te/client/data_classes/datapoints_subscriptions.py	71.42%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2426   +/-   ##
=======================================
  Coverage   48.83%   48.84%           
=======================================
  Files         191      191           
  Lines       26142    26149    +7     
=======================================
+ Hits        12767    12773    +6     
- Misses      13375    13376    +1     

Files with missing lines	Coverage Δ
...te/client/data_classes/datapoints_subscriptions.py	33.05% <71.42%> (+3.31%)	⬆️

... and 2 files with indirect coverage changes

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Invalidated by push of 53d62b9

[haakonvt]

🦄