hold evalLock while computing tostring

[davidkoski]

Proposed changes

• tostring internally calls eval, which is not thread safe

Observed while investigating ml-explore/mlx-swift-lm#291

(lldb) bt
* thread #12, name = '[Swift Testing] test gemma3nAttentionTest2() - running (Task 160)', queue = 'com.apple.root.user-initiated-qos.cooperative'
    frame #0: 0x000000018a037ae0 IOKit`iokit_user_client_trap + 8
    frame #1: 0x0000000192317184 IOSurface`-[IOSurfaceSharedEvent waitUntilSignaledValue:timeoutMS:] + 72
    frame #2: 0x00000001105f3464 MLXLMTests`bool NS::Object::sendMessage<bool, unsigned long long, unsigned long long>(pObj=0x0000000a6911ebc0, selector="waitUntilSignaledValue:timeoutMS:", args=1, args=18446744073709551615) at NSObject.hpp:216:16
    frame #3: 0x00000001105f3438 MLXLMTests`MTL::SharedEvent::waitUntilSignaledValue(this=0x0000000a6911ebc0, value=1, milliseconds=18446744073709551615) at MTLEvent.hpp:154:12 [inlined]
    frame #4: 0x00000001105f3404 MLXLMTests`mlx::core::Event::wait(this=0x0000000a68b23738) at event.cpp:25:14
    frame #5: 0x000000010fbb8f80 MLXLMTests`mlx::core::array::wait(this=0x0000000170029a30) + 124
    frame #6: 0x0000000110937080 MLXLMTests`mlx::core::eval(outputs=size=0) at transforms.cpp:324:40
    frame #7: 0x000000010fbb9044 MLXLMTests`mlx::core::array::eval(this=0x0000000170029bc0) + 156
    frame #8: 0x00000001109625d0 MLXLMTests`mlx::core::operator<<(os=0x0000000170029be0, a=array @ 0x0000000170029bc0) + 56
  * frame #9: 0x000000010fafa4c4 MLXLMTests`::mlx_array_tostring(str_=<unavailable>, arr=<unavailable>) + 104
    frame #10: 0x000000010fa56190 MLXLMTests`MLXArray.description.getter() + 244

Checklist

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• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)