fix(mcp): preserve existing metadata

[EfeDurmaz16]

Summary

Preserves existing MCP _meta fields when x402 attaches payment request or settlement metadata.

MCP _meta can carry non-payment context such as trace IDs, auth hints, evidence handles, or other transport metadata. The current helpers and server wrapper replace _meta when adding x402 metadata, which can drop that context.

Changes

• Preserve existing request _meta in attachPaymentToMeta
• Preserve existing result _meta in attachPaymentResponseToMeta
• Preserve handler-provided _meta when createPaymentWrapper attaches settlement response metadata
• Add unit coverage for all three preservation paths

Verification

• pnpm --dir typescript --filter @x402/mcp exec vitest run test/unit/utils.test.ts test/unit/server.test.ts
• pnpm --dir typescript --filter @x402/mcp lint:check
• git diff --check

Note: the full pnpm --dir typescript --filter @x402/mcp test suite currently fails in test/unit/client.test.ts because @x402/core/schemas cannot be resolved from src/client/x402MCPClient.ts in this local workspace setup. The two test files covering this change pass.

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