fix: call onerror callback before throwing transport errors

[ctonneslan]

Summary

• Several transport implementations threw errors without first invoking the onerror callback, causing errors to be silently swallowed
• Adds try-catch with onerror reporting to send() methods in:
  • StdioClientTransport (client stdio)
  • StdioServerTransport (server stdio)
  • WebSocketClientTransport (client websocket)
  • StreamableHTTPServerTransport (server streamable HTTP)

Fixes #1395

Test plan

• Existing transport tests still pass
• Errors thrown by send() are reported via onerror before propagating
• Error objects are preserved (same instance thrown and reported)

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Latest commit: a6d1460

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commit: a6d1460

[travisbreaks]

The core change is correct: calling onerror before throwing/rejecting ensures error handlers see the error before promise rejection propagates. This matches how Node.js streams work (emit 'error' before destroy).

Overlap note: PR #1705 by the same author includes all of these onerror additions plus the close() re-entrancy guard. If #1705 lands, this PR is redundant. Worth coordinating with the maintainers on which to merge first, or consolidating.

One concern on the event store wrapping:

try {
    primingEventId = await this._eventStore.storeEvent(streamId, {} as JSONRPCMessage);
} catch (error) {
    const err = error as Error;
    this.onerror?.(err);
    throw err;
}

If this.onerror itself throws, the original error is lost. A safer pattern:

} catch (error) {
    const err = error as Error;
    try { this.onerror?.(err); } catch { /* handler error should not mask transport error */ }
    throw err;
}

This is defensive, but onerror is a user-provided callback so it could do anything.

[ctonneslan]

Good catch on the defensive wrapping — pushed a fix. All 10 onerror call sites across the 4 transport files are now wrapped in try/catch so a throwing user callback can't mask the original transport error.

Re: overlap with #1705 — this PR is actually complementary, not redundant. #1705 fixes close() re-entrancy, this one fixes send() error handling. They touch different code paths. Updated #1705's description to note the relationship.