parser: Use encapsulated string constants for non-file sources

[EricCousineau-TRI]

🦟 Bug fix

Fixes #536, follow-up to #537
Downstream: gazebosim/gz-sim#794

Summary

Takes prior revision of #537.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed) - N/A
• Updated migration guide (as needed) - N/A
• Code check passed (In source directory, run sh tools/code_check.sh)
• All tests passed (See
  test coverage)
• While waiting for a review on your PR, please help review
  another open pull request
  to support the maintainers

Note to maintainers: Remember to use Squash-Merge

[azeey]

I think this will change behavior in a stable release. The string "data-string" is used in a stable release of ign-gazebo. Can we target this to main?

[chapulina]

I think this will change behavior in a stable release.

True. How about keeping the string as data-string on this branch and updating it to <data-string> on main?

[EricCousineau-TRI]

Done! Retargeted main

Meta: I'm a wee bit confused on release process and expected stability for version branches and all that. Y'all got a page that explains that part?

[aaronchongth]

Thanks for quick fixes!

Meta: I'm a wee bit confused on release process and expected stability for version branches and all that. Y'all got a page that explains that part?

I would be curious to learn more about this too

[EricCousineau-TRI]

Per VC, ABI checker failure may be due to statefulness; @azeey will try to re-run; simplest mitigation may be to ignore CI results and merge pending approval.
https://build.osrfoundation.org/job/sdformat-abichecker-any_to_any-ubuntu_auto-amd64/1162/console

Most likely, job got queued upon initial PR, but didn't go away when branch was retargeted.