🐛 Bug Fix: Fix upsert not working after external end() call

Description

This pull request fixes a bug where calling upsert() after ending the previous upsert instance externally via MyCallable.end() fails to create a new instance.

The issue occurred because the internal $upsertPromise variable was not being cleared when end() was called externally, causing subsequent upsert() calls to attempt updating a non-existent instance instead of creating a new one.

Root Cause

When an upsert instance is created, it stores the promise in $upsertPromise. This variable is used to determine whether to create a new instance or update an existing one.

The problem:

• When ending via the component's internal call.end() → $upsertPromise is cleared ✅
• When ending via external MyCallable.end() → $upsertPromise was not cleared ❌

This caused the next upsert() call to:

1. Find $upsertPromise still exists
2. Try to update the existing instance
3. Fail to find it in the stack (because it was already removed)
4. Return the old promise without creating a new instance

Solution

Modified the end() method to clear $upsertPromise when:

• Ending all calls (no specific promise target), OR
• Ending a specific promise that matches $upsertPromise

Changes

• Fixed: react-call/src/createCallable/index.tsx - Clear $upsertPromise in end() method
• Added: tests/src/upsert.test.tsx - Test case to reproduce and verify the fix

All tests now pass: ✅ 5 passed (including the new test case)

Related Issues

Fixes #71

Checklist

• Bug fix implemented
• Test case added to prevent regression
• All existing tests still pass
• No breaking changes

Notes for Reviewers

• The fix is minimal and focused on the root cause
• Fully backward compatible - no API changes
• All 5 upsert tests pass, including the new regression test