fix(cli): import yaml lazily so the CLI works without the yaml extra

[uttam12331]

Description

Fixes the CLI dying at import time when litestar is installed without pyyaml:

$ litestar -h
...
  File ".../litestar/cli/commands/schema.py", line 13, in <module>
    from yaml import dump as dump_yaml
ModuleNotFoundError: No module named 'yaml'

litestar/cli/commands/schema.py imported yaml at module level, and it is imported eagerly via litestar.cli.main — so every CLI invocation failed. pyyaml only arrives transitively with the standard extras (via uvicorn), while the docs promise the CLI and its hard dependencies are included by default.

The yaml import now happens inside the YAML-export branch of _generate_openapi_schema, raising MissingDependencyException("pyyaml", extra="yaml") when missing — the same pattern used for other optional dependencies. Exporting to .json (and every other CLI command) no longer needs pyyaml; exporting to .yaml/.yml gives an actionable install hint instead of a bare traceback.

Verified in a venv without pyyaml: litestar --help fails on main with the reported traceback and works with this change. Added test_cli_importable_without_pyyaml (simulates pyyaml's absence via sys.modules), which fails on main; the full test_schema_commands.py suite passes (14 passed).

Disclosure per the contribution guidelines: this fix was developed with AI assistance (Claude Code), with the behavior verified end-to-end locally as described above.

Closes

Closes #4449

---

📚 Documentation preview 📚: https://litestar-org.github.io/litestar-docs-preview/4929

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.30%. Comparing base (bd18fd4) to head (f48acb3).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4929   +/-   ##
=======================================
  Coverage   67.29%   67.30%           
=======================================
  Files         293      293           
  Lines       15228    15230    +2     
  Branches     1728     1728           
=======================================
+ Hits        10248    10250    +2     
  Misses       4833     4833           
  Partials      147      147           

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[sobolevn]

Great, thank you!