fix(python): unwrap alias types in pagination results type annotation

[iamnamananand996]

Description

Note: This fix has already been merged to main as part of v5.3.1. After rebasing, this PR now has zero diff against main and can be closed.

Fixes a mypy type error in generated pagination endpoints when the results property type is a named alias to a list (e.g., AgreementsList = List[Agreement]).

Discovered via fern-demo/docusign-python-sdk#4 where mypy reported:

Argument "items" to "SyncPager" has incompatible type "list[Agreement] | None"; expected "list[list[Agreement]] | None"

Changes Made

All changes below were independently merged to main as v5.3.1 before this PR could be merged. The branch is now empty.

• endpoint_function_generator.py: Extended _unwrap_container_types to handle named type references that resolve to alias types. When the pagination results property is a named alias to a list/set, the method now resolves the alias declaration, checks if its resolved type is a container, and recursively unwraps it — producing SyncPager[Agreement, ...] instead of SyncPager[AgreementsList, ...].
• versions.yml: Added changelog entry for v5.3.1.

Testing

• Ran seed test --generator python-sdk --fixture pagination — all 3 configurations pass with no generated code changes
• Pre-commit hooks pass (ruff, ruff-format, etc.)
• pnpm run check passes
• All CI checks green (78 pass, including all seed tests and ETE)
• No new seed fixture added — existing pagination fixtures don't use alias types for results properties. The fix was validated against the real-world docusign SDK.

Reviewer Checklist

• This PR can be closed — the equivalent fix is already on main (v5.3.1)
• Verify that the merged v5.3.1 on main correctly handles chained aliases (not just one level of resolution)
• Consider whether non-container alias resolved types (e.g., alias to a primitive or named type) need handling, or if falling through to return the original type is correct

Link to Devin session: https://app.devin.ai/sessions/c854539c0cc24a0d9627ebf03398dc1c
Requested by: @iamnamananand996

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