fix(core): forward cause from the SdkError data slot to Error.cause

[xiangnuans]

Closes #2657

Problem

classifyNetworkError (and other call sites) construct an SdkError with the underlying error in the data slot as { cause }:

new SdkError(SdkErrorCode.EraNegotiationFailed, `Version negotiation probe failed: ${describeError(error)}`, { cause: error })

But SdkError's third parameter is data, not ErrorOptions, and the constructor called super(message) with no options — so Error.cause stayed undefined and the { cause } object just landed on this.data. Anything walking the standard .cause chain (pino's default err serializer, Sentry's exception linking) stops at the SdkError, so the root network error is lost:

ENOTFOUND / ECONNREFUSED / ETIMEDOUT  →  all render as just "fetch failed"

Fix

Per the issue's preferred option 1 (safer, since the mismatch is silent at every call site), SdkError now forwards cause to super() when its data carries one, so the chain is unbroken:

SdkError: Version negotiation probe failed: fetch failed
  └─ TypeError: fetch failed
       └─ Error: getaddrinfo ENOTFOUND does-not-resolve.invalid

• The full data object is still retained on this.data (backwards compatible — .data.cause keeps working).
• SdkHttpError's { status, statusText } payload has no cause, so it is unaffected.
• Covers any other call site that hands { cause } to the data slot.

Tests

Added packages/core-internal/test/errors/sdkErrorCause.test.ts:

• cause from the data slot reaches Error.cause (fails on main without the fix)
• the full object is still retained on .data
• no cause / no data → .cause stays undefined
• an SdkHttpError status payload is not treated as a cause

pnpm --filter @modelcontextprotocol/core-internal test (1452 passing), typecheck, and lint all clean.

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