fix(mcp): persist request-level index options for sync

[Showiix]

Summary

Fixes #359.

MCP index_codebase request-level options such as customExtensions and ignorePatterns affect the file set used for indexing, but they were not persisted in the MCP codebase snapshot. After an MCP restart, background sync could rebuild a FileSynchronizer without those original options, causing custom-extension files to stop being tracked or request-ignored files to be considered again.

This PR:

• stores request-level customExtensions and ignorePatterns on the codebase snapshot entry
• preserves those options across indexing progress, indexed, failed, and interrupted-indexing state transitions
• restores those options when MCP background sync calls reindexByChange() after restart
• keeps the options request/codebase-scoped and does not mutate global Context extension or ignore-pattern state

Tests

• pnpm --filter @zilliz/claude-context-core test
• pnpm --filter @zilliz/claude-context-mcp test
• pnpm --filter @zilliz/claude-context-core build
• pnpm typecheck
• pnpm --filter @zilliz/claude-context-mcp build
• git diff --check