nextchanges: Enforce changelog fragment format

[janniklasrose]

Changes

• Update tools/validate_nextchanges.py to be more opinionated:
  • Leading bullet * required
  • All #123 numbers are expanded github links (issues and PRs)
  • Ends in a bullet and has trailing PR in parentheses.
    • Trailing PR link is skipped if no PR is associated to the branch/commit (e.g. while locally developing)
• Remove tools/update_github_links.py and have all link validation be done in tools/validate_nextchanges.py
• .github/workflows/changelog-preview.yml workflow (runs on PRs and pushes to main) does strict validation on the PR being added to new fragments

Why

We've been skipping PR numbers/links for a few releases. This enforces it.

Tests

Added 4 test.md files (see https://github.com/databricks/cli/tree/99d5458b3248482f21e2b5f12568bbd8d74d6c44/.nextchanges/cli)

Workflow validation of test files added to this PR:

/home/runner/work/cli/cli/.nextchanges/cli/test1.md: must start with a "* " bullet marker
/home/runner/work/cli/cli/.nextchanges/cli/test2.md: unexpanded reference #123: write it as a markdown link, e.g. [#123](https://github.com/databricks/cli/pull/123)
/home/runner/work/cli/cli/.nextchanges/cli/test3.md: trailing PR link #6394 must include the PR that added this fragment (#6395)

When fixing the PR attribution in test3.md to 6395 locally and running task changelog-preview it passes (because locally the test calls gh and associates the branch to an open PR). If running locally before opening a PR (or quickly renaming branch) the test passes since we don't enforce PR link logic there.

Valid entries look like this (task changelog-preview):

## Release v1.15.0 (2026-08-27)

### CLI

 * This is an entry. ([#6395](https://github.com/databricks/cli/pull/6395))
 * This is another entry. This feature was developed in a stack of PRs. ([#6394](https://github.com/databricks/cli/pull/6394), [#6395](https://github.com/databricks/cli/pull/6395))

[github-actions]

Waiting for approval

Based on git history, these people are best suited to review:

• @pietern -- recent work in ./
• @denik -- recent work in ./, tools/

Eligible reviewers: @andrewnester, @anton-107, @lennartkats-db, @rclarey, @renaudhartert-db, @rugpanov, @shreyas-goenka, @simonfaltum

_{Suggestions based on git history. See OWNERS for ownership rules.}

[eng-dev-ecosystem-bot]

Integration test report

Commit: 6b463c6

Run: 33077006761

	Env	💚​RECOVERED	🙈​SKIP	✅​pass	🙈​skip	Time
💚​	aws linux	1	4	274	1191	4:09
💚​	aws windows	1	4	276	1189	3:29
💚​	azure linux	1	4	273	1191	4:01
💚​	azure windows	1	4	275	1189	3:30
💚​	gcp linux	1	4	274	1191	4:14
💚​	gcp windows	1	4	276	1189	3:30

	Test Name	aws linux	aws windows	azure linux	azure windows	gcp linux	gcp windows
💚​	TestAccept	💚​R	💚​R	💚​R	💚​R	💚​R	💚​R
🙈​	TestAccept/bundle/invariant/no_drift	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S
🙈​	TestAccept/bundle/resources/vector_search_endpoints/drift/recreated_same_name	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S
🙈​	TestAccept/bundle/resources/vector_search_indexes/recreate/embedding_dimension	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S
🙈​	TestAccept/ssh/connection	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S

Top 3 slowest tests (at least 2 minutes):

duration	env	testname
3:26	azure windows	TestAccept
3:25	aws windows	TestAccept
3:15	gcp windows	TestAccept