Release 26.9

[Sripadvallabh]

Summary

Prepare the pyATS 26.9 release by merging release_26.9 into main.

Includes release version updates and changes accumulated on the release branch, with September 2026 changelogs where applicable.

[Sripadvallabh]

CI failure review for the 26.9 release: the Actions failures are caused by an older public dependency and are unrelated to a defect in the matching 26.9 release code.

The latest failed PR job installs unicon-26.8, then runs the new 26.9 connection-dialog override tests. Unicon 26.8 lacks _run_replacement_state_detection, _connection_dialog_kwargs, and the new connection_dialog argument. The resulting AttributeError/TypeError failures and default-dialog behavior follow from that dependency mismatch.

These tests are new in this PR, so they are not being described as unchanged or unrelated test files. The failure cause is that Actions tests them against the previous public core package before the 26.9 packages are published. The affected test file and IOS XE provider match the internal release sources exactly.

Release validation: Linux #472 and Linux extended #19 both completed successfully with matching release_26.9 sources. Their logs show the IOS version-learning test, all 23 plugin connection-dialog override tests, and the Moonshine HA recovery test passing across Python 3.10–3.14. The internal release checkouts were Unicon 94e3d414b9ecb2f2a3f7d318835b71e275a106ae and Unicon Plugins 6af53ca172a99a8270bd641358ef123a0bbee0cc.

The macOS final build #268 passed with plugin tests disabled; it is not evidence of a complete passing macOS plugin suite.

The release runbook acknowledges external CI version conflicts before package publication. Merging with the release lead's explicit exception for unrelated CI dependency failures; the failed Actions run and fail-fast cancellations are not being represented as passing tests.