feat: add types and helper function for context propagation

[timl3136]

What changed?
Add types and helper function for context propagation

Why?
This is first commit in stacked branches aiming to implement context propagation in python client

How did you test it?
unit test

Potential risks

Release notes

Documentation Changes

[codecov]

Codecov Report

❌ Patch coverage is 74.32432% with 19 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cadence/_internal/context_propagation.py	72.46%	14 Missing and 5 partials ⚠️

Files with missing lines	Coverage Δ
cadence/context.py	100.00% <100.00%> (ø)
cadence/error.py	95.04% <100.00%> (+0.04%)	⬆️
cadence/_internal/context_propagation.py	72.46% <72.46%> (ø)

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[gitar-bot]

Code Review ✅ Approved 2 resolved / 2 findings

Adds context propagation primitives including the ContextPropagator protocol and propagation engine. Addresses missing unit tests and refines error handling to prevent double-wrapping of ContextPropagationError.

✅ 2 resolved

✅ Quality: New context-propagation primitives have no unit tests

📄 cadence/_internal/context_propagation.py:1-15 📄 cadence/context.py:1-15
These new primitives contain non-trivial logic (mapping/type validation, duplicate-header detection, ExitStack-based scope teardown with nested cleanup error handling) but ship with no tests and no callers. Add unit tests covering: duplicate header rejection, non-mapping/non-bytes inject returns, extract failure triggering stack.close(), and cleanup errors surfacing as ContextPropagationError. This locks in the intended error semantics before consumers depend on them.

✅ Quality: inject() error handling double-wraps ContextPropagationError

📄 cadence/_internal/context_propagation.py:33-38 📄 cadence/_internal/context_propagation.py:90-99
In context_propagation_scope, the extract loop deliberately re-raises ContextPropagationError before the generic except Exception to avoid double-wrapping. inject_context_fields does not do this: a ContextPropagationError raised inside a propagator's inject() is caught by the bare except Exception and re-wrapped into a generic "inject failed" error, obscuring the original message. For consistency, add an except ContextPropagationError: raise clause before the broad handler in the inject path.

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[gitar-bot]

Code Review ✅ Approved 2 resolved / 2 findings

Adds context propagation primitives including the ContextPropagator protocol and propagation engine. Addresses missing unit tests and refines error handling to prevent double-wrapping of ContextPropagationError.

✅ 2 resolved

✅ Quality: New context-propagation primitives have no unit tests

📄 cadence/_internal/context_propagation.py:1-15 📄 cadence/context.py:1-15
These new primitives contain non-trivial logic (mapping/type validation, duplicate-header detection, ExitStack-based scope teardown with nested cleanup error handling) but ship with no tests and no callers. Add unit tests covering: duplicate header rejection, non-mapping/non-bytes inject returns, extract failure triggering stack.close(), and cleanup errors surfacing as ContextPropagationError. This locks in the intended error semantics before consumers depend on them.

✅ Quality: inject() error handling double-wraps ContextPropagationError

📄 cadence/_internal/context_propagation.py:33-38 📄 cadence/_internal/context_propagation.py:90-99
In context_propagation_scope, the extract loop deliberately re-raises ContextPropagationError before the generic except Exception to avoid double-wrapping. inject_context_fields does not do this: a ContextPropagationError raised inside a propagator's inject() is caught by the bare except Exception and re-wrapped into a generic "inject failed" error, obscuring the original message. For consistency, add an except ContextPropagationError: raise clause before the broad handler in the inject path.

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