fix(client): put the probe's network failure on Error.cause, not data

[likalight]

Fixes #2657.

The bug

SdkError's third constructor parameter is data, not ErrorOptions:

constructor(
    public readonly code: SdkErrorCode,
    message: string,
    public readonly data?: unknown
) {
    super(message);   // no options forwarded

So classifyNetworkError passing { cause: error } as the third argument
never reached Error.cause. The underlying failure was stranded at
error.data.cause, where nothing walking the standard .cause chain —
loggers, error reporters, util.inspect — will find it. Connecting to an
unreachable host surfaced an SdkError whose .cause was undefined, with
the TypeError: fetch failed and the DNS error that actually names the
problem both invisible to normal error handling.

The fix

Set cause on the instance with the same non-enumerable descriptor the
Error constructor would have produced.

I grepped the repo: probeClassifier.ts:314 was the only site passing
{ cause } into an SdkError data slot, so this is self-contained.

One thing worth your call

test/client/probeAuthSeam.test.ts had an assertion pinning the old
behaviour:

expect(((out.error as SdkError).data as { cause?: unknown }).cause).toBe(netError);

I updated it to assert .cause instead, on the reading that it was codifying
the bug rather than a contract you intend to keep. If data.cause is
load-bearing for downstream consumers, say so and I'll set both — the
one-line alternative is to keep passing { cause: error } as data in
addition to setting the real cause. Happy to go either way.

Verification

• Added a regression test in probeClassifier.test.ts asserting the full
  chain resolves (SdkError → TypeError: fetch failed → ENOTFOUND).
  Verified it fails without the source change:
  AssertionError: expected undefined to be TypeError: fetch failed
• packages/client: 805/805 tests pass
• tsc --noEmit: clean
• prettier --check: clean on all touched files

Note: pnpm lint:all fails on this branch, but it fails identically on a
clean checkout of main — a sync:snippets docs drift unrelated to this
change.

[changeset-bot]

🦋 Changeset detected

Latest commit: faba11c

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

This PR includes changesets to release 6 packages

Name	Type
@modelcontextprotocol/client	Patch
@modelcontextprotocol/core	Patch
@modelcontextprotocol/server	Patch
@modelcontextprotocol/server-legacy	Patch
@modelcontextprotocol/codemod	Patch
@modelcontextprotocol/core-internal	Patch

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commit: faba11c