sdk: tighten ReadableSpan.attributes to non-Optional Mapping (#4569)

[WatchTree-19]

Description

ReadableSpan.attributes is annotated as types.Attributes, which resolves to Optional[Mapping[str, AttributeValue]]. The implementation always returns MappingProxyType(self._attributes or {}) - the or {} fallback ensures we never return None at runtime.

Pyright / Pylance flags Object of type "None" is not subscriptable on span.attributes["key"], forcing assert span.attributes is not None boilerplate at every call site.

This PR tightens the return annotation on ReadableSpan.attributes to the non-Optional Mapping[str, types.AttributeValue]. Implementation unchanged. Mapping is already imported in the file.

Scope:

• ReadableSpan.attributes only. Inherited by Span and _Span, no further changes needed.
• Event.attributes and Link.attributes return self._attributes directly (which can be None) and stay Optional.
• types.Attributes global alias is unchanged for the same reason.

Per the comment thread on #4569 from @exekis and @tekumara, the implementation has never returned None from ReadableSpan.attributes, so this is type-system tightening rather than a behaviour change. Technically a breaking change for any caller explicitly handling None on the return; in practice there shouldn't be any.

Fixes #4569

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• patched file parses cleanly via python -m ast
• existing call sites are unchanged, only the return-type annotation is narrower
• have not run the project test suite locally; happy to chase a green CI here if anything trips

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: WatchTree-19 / name: WatchTree-19 (2fb0968, 97d0d2a)