fix: move TraceContext.spans from class to instance attribute

[ashtarkb]

What does this PR do?

The spans list in TraceContext was defined as a class-level attribute, causing all TraceContext instances to share the same list. This led to trace corruption when handling concurrent requests, as spans from different requests would be mixed together.

The bug:
class TraceContext:
spans: list[Span] = [] # Shared by ALL instances!

The fix:
Moving spans initialization to __init__ ensures each TraceContext instance has its own isolated spans list:

class TraceContext:
    def __init__(self, logger: BackgroundLogger, trace_id: str):
        self.logger = logger
        self.trace_id = trace_id
        self.spans: list[Span] = []  # Instance-levelCloses #4431

Test Plan

1. Deploy LlamaStack with telemetry enabled (OpenTelemetry Collector)
2. Send 3 concurrent requests using the reproduction script from TraceContext uses shared mutable class attribute causing trace corruption #4431
3. Verify each request creates its own isolated trace (3 traces instead of 1 merged trace with 34 spans)

Before fix: 3 concurrent requests → 1 merged trace with 34 spans
After fix: 3 concurrent requests → 3 separate traces with ~10 spans each

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[nathan-weinberg]

This is a good catch - thanks for the fix!