refactor: extract DistributedTraceMiddleware into its own file

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What does this MR do?

Extracts the DistributedTraceMiddleware class from the base middleware file into its own dedicated file, along with its corresponding tests.

Related issues

Closes #860 (closed)

Implementation details

• Created new file ai_gateway/api/middleware/distributed_trace.py for the middleware
• Moved DistributedTraceMiddleware class to the new file
• Created new test file tests/api/middleware/test_distributed_trace.py
• Moved related tests to the new test file
• Updated imports across the codebase

MR acceptance checklist

• All tests pass
• Code style checks pass