fix(changelog): include non-leading release metadata - #1472

Purpose

Fixes #1471. The default changelog templates omit a breaking-change description or release notice when it belongs to a commit that is not the first commit in its category.

Rationale

The existing dotted attribute path 1.0 selects only index 0 from each category. The templates now flatten every category's commit list before filtering for breaking_descriptions and release_notices.

How did you test?

• Added a regression fixture where a newer ordinary commit precedes both a breaking commit and a release-notice commit in their respective categories.
• Verified the default Markdown and reStructuredText changelogs both include the omitted sections and descriptions.
• Ran pytest tests/unit/semantic_release/changelog/test_default_changelog.py -q: 34 passed.
• Ran Ruff format and lint checks on the changed tests.
• Ran pytest tests/unit/semantic_release/changelog -m unit -q: 139 passed, 15 skipped; 7 pre-existing Windows newline expectation failures remain in test_release_notes.py, outside this change's code path.

How to Verify

1. Run pytest tests/unit/semantic_release/changelog/test_default_changelog.py -k nonleading -vv.
2. Confirm both md and rst cases pass.

PR Completion Checklist

• Reviewed and followed the Contributor Guidelines.
• Validation pipeline succeeds.
• Commit follows the Conventional Commits standard.
• Appropriate unit tests added.
• No end-to-end test is needed because this is isolated default-template rendering behavior covered at the unit layer.
• No documentation change is needed.