Fix changelog extraction to support unreleased content in release pipeline

[Copilot]

The release pipeline for v0.3.3 failed because changelog_utils.py expected a versioned changelog entry (## [v0.3.3]) but changes were in [Unreleased] per Keep a Changelog conventions.

Changes

• Enhanced extract_changelog_content() to fall back to [Unreleased] section when requested version doesn't exist
  • Maintains existing behavior: prefers versioned sections when available
  • Raises ValueError only if both version and unreleased content are missing/empty
• Added 3 test cases covering fallback extraction, empty unreleased handling, and version preference
• Updated CHANGELOG.md with fix entry

Behavior

# Before: version not found → ValueError
extract_changelog_content(Path('CHANGELOG.md'), 'v0.3.3')
# Error: Version v0.3.3 not found in CHANGELOG.md

# After: version not found → extracts from [Unreleased]
extract_changelog_content(Path('CHANGELOG.md'), 'v0.3.3')
# Returns content from [Unreleased] section

# Existing versions still work unchanged
extract_changelog_content(Path('CHANGELOG.md'), 'v0.3.0')
# Returns content from [v0.3.0] section

This unblocks v0.3.3 release. Once merged, tag and push v0.3.3 to trigger the release workflow.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Investigate release pipeline failure for v0.3.3 and suggest next steps</issue_title>
<issue_description>## Summary

The release pipeline failed during the extraction of changelog content for version v0.3.3. The error indicates that the specified version was not found in CHANGELOG.md, causing the process to exit with code 1.

Error log excerpt

CHANGELOG_CONTENT=$(python3 scripts/changelog_utils.py extract "v0.3.3")
CHANGELOG_CONTENT=$(python3 scripts/changelog_utils.py extract "v0.3.3")
echo "content<<EOF" >> $GITHUB_OUTPUT
echo "$CHANGELOG_CONTENT" >> $GITHUB_OUTPUT
echo "EOF" >> $GITHUB_OUTPUT
shell: /usr/bin/bash -e {0}
env:
  CARGO_TERM_COLOR: always
Error: Version v0.3.3 not found in CHANGELOG.md
Error: Process completed with exit code 1.

Requested actions

• Investigate why version v0.3.3 is missing from CHANGELOG.md or if there is a parsing issue.
• Determine if this is a configuration, documentation, or automation error.
• Suggest next steps to unblock the current release pipeline.

Context

This is impacting the current release process, so guidance on a quick resolution or workaround is appreciated.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Investigate release pipeline failure for v0.3.3 and suggest next steps #471

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[Mossaka]

The security audit is irrelevant to this PR AND should be fixed from another task.