fix: restore search score threshold slider

[daniel-lxs]

Description

This PR restores the search score threshold slider feature that was accidentally deleted in PR #5041

Problem

1. The search score threshold slider was removed from the CodeIndexPopover component
2. When the slider existed, setting the value to 0 would cause it to reset to the default value (0.4) upon saving
3. The value was not being properly persisted in the global state

Changes Made

• Added missing field to WebviewMessage type: Added codebaseIndexSearchMinScore to the codeIndexSettings interface
• Fixed state persistence: Updated saveCodeIndexSettingsAtomic handler to include codebaseIndexSearchMinScore when saving to global state
• Initialized field in ExtensionStateContext: Added codebaseIndexSearchMinScore: undefined to the default state
• Fixed falsy value handling: Changed all logical OR operators to nullish coalescing operators in CodeIndexPopover.tsx to properly handle 0 values
• Verified schema: Confirmed that codebaseIndexSearchMinScore is already defined in the codebaseIndexConfigSchema

Testing

• All existing tests pass
• Manually tested setting the search score threshold to 0
• Verified the value persists after saving settings
• Confirmed the slider displays the correct value after reopening the popover

Related Issues

• References PR feat: add user-configurable search score threshold slider for semantic search (#5027) #5041 where this feature was accidentally removed

Checklist

• Code follows project style guidelines
• Self-review completed
• No breaking changes
• All tests pass

---

Important

Restores search score threshold slider and fixes persistence of codebaseIndexSearchMinScore in CodeIndexPopover and global state.

• Behavior:
  • Restores search score threshold slider in CodeIndexPopover.tsx.
  • Fixes persistence of codebaseIndexSearchMinScore in global state via saveCodeIndexSettingsAtomic in webviewMessageHandler.ts.
  • Handles 0 values correctly using nullish coalescing in CodeIndexPopover.tsx.
• State Management:
  • Adds codebaseIndexSearchMinScore to WebviewMessage type in WebviewMessage.ts.
  • Initializes codebaseIndexSearchMinScore in ExtensionStateContext.tsx.
• Schema:
  • Confirms codebaseIndexSearchMinScore is defined in codebaseIndexConfigSchema in codebase-index.ts.

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[delve-auditor]

✅ No security or compliance issues detected. Reviewed everything up to ec55274.

Security Overview

• 🔎 Scanned files: 5 changed file(s)

Detected Code Changes

Change Type	Relevant files
Bug Fix	► codebase-index.ts     Add search score constants ► webviewMessageHandler.ts     Add codebaseIndexSearchMinScore to settings handler ► WebviewMessage.ts     Add codebaseIndexSearchMinScore type ► CodeIndexPopover.tsx     Restore search score threshold slider and value persistence ► ExtensionStateContext.tsx     Initialize codebaseIndexSearchMinScore

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