fix: accept null tool call arguments

[2830500285]

Fixes #2012.

Summary

• Accept null for tools/call arguments in the core request schema and normalize it to the same output as omitted arguments.
• Add regression coverage for direct schema parsing and the high-level McpServer tools/call path.

Validation

• Ran a targeted Zod runtime check for the updated schema expression: { arguments: null } parses successfully and normalizes to omitted, while non-record values still fail.
• Attempted corepack pnpm install --frozen-lockfile and corepack pnpm install --frozen-lockfile --ignore-scripts; dependency linking did not complete in this Windows environment, so Vitest could not be run locally.

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Latest commit: 86d827e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@modelcontextprotocol/core	Patch
@modelcontextprotocol/server	Patch

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