feat(analytics): identify MCP gateway traffic

[KillerQueen-Z]

Summary

• mark requests to BlockRun gateways with User-Agent: blockrun-mcp/<version>
• override the SDK blockrun-ts/* identifier at the MCP product boundary
• leave third-party requests unchanged
• guard against installing the global fetch wrapper more than once

This makes MCP traffic independently queryable through the existing client_source usage-log field, including Base and Solana gateway calls.

Validation

• npm run typecheck
• npm run build
• npm test — 151 tests passed
• npm pack --dry-run --json

Deployment

Publish the next @blockrun/mcp version after merge. New MCP calls will then appear as blockrun-mcp/<version> instead of blockrun-ts/<version>.