Improve error message when publish-docs ref is a branch not a tag

[shahar1]

When running breeze workflow-run publish-docs --ref providers/YYYY-MM-DD-docs ... without --skip-tag-validation, the error message was misleading:

Error: Ref providers/2026-04-26-docs does not exists in repo apache/airflow

This is confusing because the ref does exist — just as a branch, not a tag. The new behaviour:

1. When the tag check fails, additionally checks if the ref exists as a branch via gh api repos/apache/airflow/git/refs/heads/{ref}.
2. If it is a branch, emits a clear, actionable message pointing to --skip-tag-validation.
3. If neither tag nor branch exists, falls back to an (also grammatically fixed) generic message.

This is particularly relevant for the "Fixing released documentation" workflow documented in dev/README_RELEASE_PROVIDERS.md, where a -docs branch is created and used as the ref.

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Was generative AI tooling used to co-author this PR?

• Yes — GitHub Copilot (Claude Sonnet 4.6)

Generated-by: GitHub Copilot (Claude Sonnet 4.6) following the guidelines

[potiuk]

A few small static-check fixes — prek run --files <both files> passes clean after these and all 4 tests still pass.

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Drafted-by: Claude Code (Opus 4.7); reviewed by @potiuk before posting