fix: add product context fallback for subchannels

[fpfp100]

fix: add product context fallback for subchannels

Ports the subchannel fallback from SDK PR: microsoft/Agent365-python#252.
Additionally, extracted a shared resolve_sub_channel helper in utils.py
and applied the same fallback to _derive_channel() in output_logging_middleware.py,
which was still reading sub_channel directly without the productContext fallback.

Also fixed a pre-existing mypy error by widening parse_retry_after type hint
from dict[str, str] to Mapping[str, str].

[Copilot]

Pull request overview

This PR fixes channel “subchannel” resolution across A365 hosting instrumentation by adding a productContext fallback (from activity.channel_data) when ChannelId.sub_channel is missing, and reuses the same resolution logic in both baggage propagation and output logging.

Changes:

• Added a shared resolve_sub_channel(activity) helper that falls back to channel_data["productContext"] (including JSON-string channel_data).
• Updated OutputLoggingMiddleware’s _derive_channel() to use the shared subchannel resolver rather than reading sub_channel directly.
• Fixed a mypy typing issue by widening parse_retry_after from dict[str, str] to Mapping[str, str], and added tests covering the new fallback behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/a365/hosting/middleware/test_output_logging_middleware.py	Adds tests verifying _derive_channel() uses productContext as a fallback and respects sub_channel precedence.
tests/a365/hosting/middleware/test_baggage_middleware.py	Adds tests ensuring baggage channel link extraction uses productContext fallback (dict + JSON string) and handles invalid JSON gracefully.
src/microsoft/opentelemetry/a365/hosting/scope_helpers/utils.py	Introduces resolve_sub_channel() and routes get_channel_pairs() through it.
src/microsoft/opentelemetry/a365/hosting/middleware/output_logging_middleware.py	Applies the shared resolve_sub_channel() logic to output logging channel derivation.
src/microsoft/opentelemetry/a365/core/exporters/utils.py	Widens parse_retry_after parameter type to Mapping[str, str] to satisfy type checking.

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