fix(ai-sdk): isolate shared model tracing

[Abhijeet Prasad (AbhiPrasad)]

resolves https://linear.app/braintrustdata/issue/BT-4980/bug-wrapaisdk-interleaved-traces-with-concurrent-calls

Before:

  span_tree:                                                                                                
  └── ai-sdk-concurrent-shared-model-root [task]                                                            
      ├── generateText [function]                                                                           
      │   input: { "marker": "MARKER_A" }                                                                   
      │   output: { "marker": "MARKER_A" }                                                                  
      │   ├── doGenerate [llm]                                                                              
      │   │   input: { "marker": "MARKER_A" }                                                               
      │   │   output: { "marker": "MARKER_A" }                                                              
      │   └── doGenerate [llm]                                                                              
      │       input: { "marker": "MARKER_B" }                                                               
      │       output: { "marker": "MARKER_B" }                                                              
      └── generateText [function]                                                                           
          input: { "marker": "MARKER_B" }                                                                   
          output: { "marker": "MARKER_B" }                                                                  

After:

  span_tree:                                                                                                
  └── ai-sdk-concurrent-shared-model-root [task]                                                            
      ├── generateText [function]                                                                           
      │   input: MARKER_A                                                                                   
      │   └── doGenerate [llm]                                                                              
      │       input: MARKER_A                                                                               
      └── generateText [function]                                                                           
          input: MARKER_B                                                                                   
          └── doGenerate [llm]                                                                              
              input: MARKER_B