Feature/graphql error handling

[ricred]

Problem
The generated client silently swallowed GraphQL errors. ExecuteAsync threw a bare exception with no structured error data — no extensions, no error codes, no locations, no paths. Subscription errors were completely lost. There was no way to access both data and errors from a partial response.
Solution
Full GraphQL error propagation following the GraphQL spec §7.1 Response Format (https://spec.graphql.org/October2021/#sec-Response):

• GraphQueryError now carries Message, Locations, Path, Extensions, and a classified ErrorCode
• GraphResult exposes Data, Errors, Extensions, HasErrors, HasData, EnsureNoErrors()
• ExecuteWithResultAsync() — opt-in Result API (no throw, get data + errors)
• ExecuteAsync() — existing API unchanged, now throws with full error details
• Subscription error handling for both WS (graphql-ws) and SSE protocols

What's New

• Add GraphQueryError with Extensions, ErrorCode, Locations, Path
• Add GraphErrorCode enum (Apollo + Hot Chocolate codes)
• Add GraphResult with Data, Errors, Extensions, HasErrors, HasData
• Add ExecuteWithResultAsync() for queries, mutations, cursor paging
• Add ProcessResponseFull() parsing full GraphQL response (data + errors + extensions)
• Handle WS error/complete messages, SSE event: error
• Add SafeProcessMessage for subscription resilience
• Add 32 error handling tests (99 total pass)
• Add Error Handling docs to README

API
Existing — backward compatible, now with rich error details:

try
{
    var result = await client.Query.Customer(id: "abc").Select().ExecuteAsync();
}
catch (GraphQueryExecutionException ex)
{
    var code = ex.Errors.First().ErrorCode;   // GraphErrorCode.Authentication
    var ext  = ex.Errors.First().Extensions;   // full server extensions
}

New — Result API, no exceptions:

var result = await client.Query.Customer(id: "abc").Select().ExecuteWithResultAsync();
if (result.HasErrors) { /* inspect result.Errors */ }
if (result.HasData)   { /* use result.Data */ }

Changes
Core — QueryExecutor.cs, GraphQueryExecute.cs, GraphBaseExecute.cs, GraphCursorPager.cs
→ ProcessResponseFull, ExecuteRawAsync, ExecuteWithResultAsync, paging variants
Types — GraphErrorCode.cs (new), GraphResult.cs (new), GraphQueryExecutionException.cs
→ Error classification (Apollo + Hot Chocolate codes), result wrapper, extensions on error
Subscriptions — WSClient.cs, SSEClient.cs, GraphSubscriptionExecute.cs, WebsocketRequestTypes.cs
→ WS error/complete messages, SSE event: error, SafeProcessMessage resilience
Tests — ErrorHandlingTests.cs (new), Query.cs (test server)
→ 32 new tests (99 total pass), error-raising test endpoints
Docs — README.md
→ Error Handling section with examples

Backward Compatibility
Fully backward compatible. ExecuteAsync() signature unchanged. New APIs are additive opt-in.