Make Zoekt force re-indexing probability configurable

What does this MR do and why?

Make the Zoekt force re-indexing probability configurable to allow tuning and provide flexibility for Self-Managed and Dedicated customers.

The REINDEXING_CHANCE_PERCENTAGE was previously hardcoded at 0.5%, which caused excessive work for very large repositories. This change makes it configurable via Admin UI settings and reduces the default to 0.25%.

Changes:

• Add zoekt_reindexing_chance_percentage setting to Search::Zoekt::Settings
• Add jsonb_accessor in ApplicationSetting with default of 0.25% (reduced from 0.5%)
• Add JSON schema validation for the new setting
• Add validation ensuring value is between 0-100
• Update IndexingTaskService to use configurable setting instead of hardcoded constant
• Add proper handling for 0% (disables random re-indexing) and 100% (always forces re-indexing)
• Add comprehensive test coverage including edge cases
• Update documentation with new setting usage

The setting can be configured via Admin > Settings > Search > Exact code search. It is positioned after "Indexing CPU to tasks multiplier" for logical grouping with other resource/performance-related indexing settings.

References

Closes #586407 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Start your GDK and ensure Zoekt is enabled
2. Navigate to Admin > Settings > Search > Exact code search
3. Locate the "Probability of random force re-indexing (%)" field (positioned after "Indexing CPU to tasks multiplier")
4. Set a value between 0 and 100:
   • 0 - Disables random force re-indexing completely
   • 0.25 - Default (0.25% probability)
   • 100 - Always forces re-indexing
5. Save changes
6. Verify the setting is persisted by refreshing the page
7. In a Rails console, verify the setting:

   ApplicationSetting.current.zoekt_reindexing_chance_percentage
   # => 0.25 (or your configured value)

8. Verify it's used in the service:

   Search::Zoekt::Settings.reindexing_chance_percentage
   # => 0.25 (or your configured value)

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