Propagate OpenTelemetry baggage through FastStream

[vrslev]

Automatically bridge OpenTelemetry baggage through FastStream telemetry middleware.

• inject the active global OpenTelemetry context on publish
• extract baggage into the active consumer span context
• materialize configured baggage span attributes
• preserve unrelated baggage across consume-to-publish flows
• support authoritative baggage removal

Tests cover value replacement, removal, downstream propagation, span attributes, and context restoration.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
unittests	99.02% <100.00%> (+0.01%)	⬆️

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Files with missing lines	Coverage Δ
microbootstrap/bootstrappers/faststream.py	100.00% <100.00%> (ø)
microbootstrap/middlewares/faststream.py	100.00% <100.00%> (ø)

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