fix: Improving handling for tag relationship when deleting assets v2

[Vitor-Avila]

SUMMARY

This is a follow up for #29117, since it was confirmed on this comment that the issue still persists.

I'm updating these relationships to viewonly=True, since there's already an after_delete hook in place to delete tag association after an asset is deleted - reference. With that in mind, handling the cascade deletion in the relationship is redundant.

Long-term, we could delete this hook and fix the relationships for a proper cascading deletion -- I'll look into that in a following PR (need to make sure that removing this hook won't impact any other logic in the application).

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

No UI changes.

TESTING INSTRUCTIONS

1. Make sure you have a dashboard and a chart with the same ID.
2. Assign a tag for both assets. Also add owners to both assets.
3. Validate that you're able to delete one of the assets.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[Vitor-Avila]

@mistercrunch @john-bodley let me know your thoughts with this approach

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.73%. Comparing base (76d897e) to head (47d4139).
Report is 1094 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #29229       +/-   ##
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+ Coverage   60.48%   83.73%   +23.24%     
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  Files        1931      518     -1413     
  Lines       76236    37522    -38714     
  Branches     8568        0     -8568     
===========================================
- Hits        46114    31419    -14695     
+ Misses      28017     6103    -21914     
+ Partials     2105        0     -2105     

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hive	48.94% <ø> (-0.22%)	⬇️
javascript	?
mysql	77.22% <ø> (?)
postgres	77.34% <ø> (?)
presto	53.57% <ø> (-0.24%)	⬇️
python	83.73% <ø> (+20.24%)	⬆️
sqlite	76.78% <ø> (?)
unit	59.21% <ø> (+1.58%)	⬆️

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[mistercrunch]

It'd be great to capture this as comment, something like

viewonly=True,  # cascading handled by tag.models.ObjectUpdater.after_delete

for all 3 intances plz. As someone reading the code, it'd be a head scratcher, and I have to say that while I worked on this bug I never knew this ObjectUpdater existed...