refactor(code-index): extract interface state and webview routing

[WebMad]

Summary

• add a feature-owned CodeIndexState read model and expose snapshot/event APIs through CodeIndexController
• introduce CodeIndexScope so the manager, state manager, and controller share a workspace lifetime
• extract code-index webview commands from the monolithic handler into CodeIndexWebviewMessageHandler
• add WebviewMessageHandlerRegistry and construct one registry per ClineProvider
• route complete code-index state updates to the webview through the controller

Verification

• repository lint passed via the commit hook
• repository type-check passed via the push hook
• focused Vitest suites passed: 7 files, 205 tests

Dependency

This PR is based on #1594 and should be reviewed/merged after it.

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[github-actions]

Review status

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Current step: Fix the failing required CI checks; awaiting-maintainer requires CI and automated review completion.

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[codecov]

Codecov Report

❌ Patch coverage is 66.95279% with 77 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...s/code-index/code-index-webview-message-handler.ts	52.41%	56 Missing and 13 partials ⚠️
src/core/webview/ClineProvider.ts	60.00%	3 Missing and 1 partial ⚠️
src/core/webview/webviewMessageHandler.ts	33.33%	1 Missing and 1 partial ⚠️
src/core/tools/CodebaseSearchTool.ts	0.00%	1 Missing ⚠️
src/services/code-index/manager.ts	50.00%	1 Missing ⚠️

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