fix(mcp): scope customExtensions to index request

[Showiix]

Summary

Fixes #356.

This makes MCP request-level customExtensions local to the current index_codebase request instead of mutating the shared long-lived Context.supportedExtensions state.

Previously, handleIndexCodebase() called Context.addCustomExtensions(...) for MCP request parameters. Since the MCP server reuses the same Context instance across indexing tasks, a custom extension requested for project A could remain enabled when indexing project B.

Changes

• Add Context.getEffectiveSupportedExtensions(...) to compute per-operation supported extensions without mutating persistent Context state.
• Allow Context.indexCodebase(...) to receive request-scoped supported extensions as an optional final argument.
• Pass the same effective extension list into FileSynchronizer and indexCodebase() so initial indexing and change tracking use consistent file inclusion rules.
• Keep Context.addCustomExtensions(...) intact for explicit persistent core API usage.
• Add regression coverage for request-scoped custom extension isolation across sequential indexes on the same Context.

Compatibility

Persistent extension configuration is unchanged:

• CUSTOM_EXTENSIONS still applies globally.
• ContextConfig.customExtensions still applies globally.
• Explicit Context.addCustomExtensions(...) calls still mutate the Context as before.

Only MCP request-level customExtensions are scoped to the current indexing task.

Tests

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