fix: sandbox creation blocked by retention validation gap and Kafka trigger cloning

[elias-ba]

Description

This PR fixes several issues blocking sandbox creation:

1. Retention validation gap - validate_dataclip_retention_period/1 used get_change/2 which only returns a value when the field is modified. When a user changes only history_retention_period, the cross-field check is skipped, allowing dataclip_retention_period > history_retention_period to be saved. This invalid state then blocks sandbox creation because the cloned settings get rejected by validation. Fixed by using get_field/2 instead.

2. Kafka trigger cloning crash - Sandbox creation crashes with Ecto.CastError for any project with a Kafka trigger. The cloning code passes the loaded %KafkaConfiguration{} struct directly to Trigger.changeset/2, but cast_embed expects a plain map. Fixed by converting the struct to a map before passing it.

3. Silent form failure - When sandbox creation fails due to a backend validation error, the form failed silently (no error message, name field emptied). Fixed by closing the modal and redirecting with a flash error, consistent with how the limiter error is handled.

4. Data migration - Aligns dataclip_retention_period to history_retention_period for projects currently in the invalid state.

Closes #4385
Closes #4440

Validation steps

1. On staging, go to qa-workflows and create a new sandbox, it should succeed (this project previously had inconsistent retention settings that blocked sandbox creation).
2. Create a sandbox from a project that has a Kafka trigger, it should succeed without crashing.
3. In any project's settings, set a dataclip retention period and try lowering the history retention period below it, the form should reject the change with a validation error.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.31%. Comparing base (9919348) to head (ea7933f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4439      +/-   ##
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- Coverage   89.37%   89.31%   -0.07%     
==========================================
  Files         425      425              
  Lines       20149    20154       +5     
==========================================
- Hits        18009    18001       -8     
- Misses       2140     2153      +13     

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