Change default parser mode to json

[mauvilsa]

What does this PR do?

• Breaking: parser_mode now defaults to json instead of yaml, so behavior no longer depends on whether pyyaml happens to be installed (it became optional in pyyaml is no longer a required dependency #970). YAML configs and YAML-syntax values, e.g. --list=[a, b] or --flag=yes, now fail unless parser_mode="yaml" is given.
• New parser_mode setting in set_parsing_settings and JSONARGPARSE_PARSER_MODE environment variable, so users can restore YAML globally without changing every parser.
• The jsonnet parser mode no longer silently accepts YAML.
• The v5 migration guide explains the change and how to keep YAML, including parser_mode="yaml", which also works in v4.

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)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (632b1dd) to head (4119ba3).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #997   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           27        27           
  Lines         9647      9657   +10     
=========================================
+ Hits          9647      9657   +10     

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[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

[greptile-apps]

Confidence Score: 4/5

[High risk] Changes the default parser mode from YAML to JSON.

The PR appears safe to merge, with a non-blocking gap in test coverage for default-JSON parsing.

Findings

1.  Default JSON behavior lacks coverage ▶

Summary

Changes the default parser mode to JSON, adds a global setting and environment override, and makes basic-type and Jsonnet loading follow the selected mode.

• Updates documentation and the v5 migration guide for the breaking default change.
• Adds mode-specific tests, although the suite-wide YAML override leaves the new default insufficiently exercised.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  A[Parser has explicit mode?] -->|Yes| B[Use explicit mode]
  A -->|No| C{Environment override?}
  C -->|Yes| D[Use JSONARGPARSE_PARSER_MODE]
  C -->|No| E[Use global setting: JSON by default]
  B --> F[Load values and configs]
  D --> F
  E --> F

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