Dump multi-line strings as YAML literal blocks

[mauvilsa]

What does this PR do?

The yaml dump format now writes multi-line strings as literal blocks, i.e. |, instead of escaping the line breaks. This makes dumps of configs that hold things like prompts, descriptions or scripts readable and editable:

# before                          # after
prompt: 'You are helpful.         prompt: |
                                    You are helpful.
  Be concise.                       Be concise.

  '

yaml_dump now uses a lazily built DefaultDumper, a subclass of yaml.SafeDumper with a str representer that requests the | style for values containing a newline. This mirrors the existing DefaultLoader pattern. Subclassing instead of registering the representer on yaml.SafeDumper keeps the change from leaking into the yaml.safe_dump calls of users, consistent with the v5 removal of the global Namespace representer.

Both YAML backends are covered. ruamel.yaml is only used for the yaml_comments format, which round-trips the PyYAML output, so it parses the block into a LiteralScalarString and re-emits it as |. The comments dump gets the same formatting, with comments still placed above the block.

Round-trips are lossless. PyYAML falls back to double-quoted style by itself when a block scalar cannot represent the value, e.g. a line with trailing spaces, non-printable characters, or a flow context. Chomping is preserved: | with a trailing newline, |- without one and |+ for several.

Before submitting

• Did you read the contributing guideline?
• If you used a coding agent, did you fully understand and validate all generated code and ensure it follows the contributing guidelines?
• Did you update the documentation? (readme and public docstrings)
• Did you write unit tests such that there is 100% coverage on related code? (required for bug fixes and new features)
• Did you verify that new and existing tests pass locally?
• [n/a] If this is a bug fix, did you verify that the tests fail without the code fix?
• Did you make sure that all changes preserve backward compatibility?
• Did you update the CHANGELOG including a pull request link? (not for typos, docs, test updates, or minor internal changes/refactors)

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (5fc71c7) to head (fd0a0d5).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #972   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           27        27           
  Lines         8721      8734   +13     
=========================================
+ Hits          8721      8734   +13     

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