fix(errors): forward SdkError causes onto Error.cause

[eddinos2]

Summary

SdkError third argument is data, but classifyNetworkError passed { cause }. That left the underlying fetch/DNS error on .data.cause instead of .cause, so pino/Sentry cause-chain walkers lost ENOTFOUND vs ECONNREFUSED detail.

• Peel { cause } out of the data bag into ErrorOptions
• Accept an explicit options?: ErrorOptions on SdkError / SdkHttpError
• Construct the era-negotiation failure with a real { cause }
• Add regression tests

Closes #2657

Test plan

• pnpm --filter @modelcontextprotocol/core-internal test (1438 passed, including new cause tests)

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commit: 93b5f60

[eddinos2]

Happy to adjust the peel heuristics if you'd rather keep cause inside data for back-compat and only document .cause — current approach peels so pino/Sentry work with zero consumer changes.