fix(explore): restored hidden field values has discarded

[justinpark]

SUMMARY

When the x-axis was changed back to a time item from a non-time control item, the values of hidden fields were not properly restored to their original state, causing them to be consistently recognized as hidden items and preventing form_data from being sent.
This commit fixes the issue and includes test code to prevent regression.

Fixes #29331

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

Before:

before--missing-hidden-fields.mov

After:

after--hidden-fields.mov

TESTING INSTRUCTIONS

1. Create line chart with temporal x-axis column and COUNT(*) metric and a specific time grain like Month
2. Switch the x-axis variable to categorical or numeric variable instead and click Update Chart
3. Switch the x-axis variable back to the original variable.

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

[rusackas]

Thank you!!!