Docs: Add custom SQL examples, validatorClass values, and failure thresholds to test API pages

[kiran1287]

Follow-up to #603. Port of open-metadata/docs-collate#797.

• Test definition create: list valid validatorClass values for custom SQL definitions and show sqlExpression in the side-panel request example (python, curl).
• Test case create: clarify that testDefinition takes the custom definition's name, add a custom SQL tab to the request example, and add a custom test definition example.
• v2.1.x-SNAPSHOT only: document threshold and thresholdUnit, add a Failure Thresholds section, and qualify the default pass condition.
• Applied to v1.13.x, v2.0.x, and v2.1.x-SNAPSHOT.

Closes #612

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Review Report

Content type: Documentation
Overall verdict: PASS
Reason: No issues found.
Reviewed revision: 139b01b
Findings: Critical=0 Major=0 Minor=0

Issues Found

No issues found.

_{Inputs are identical to an earlier review on this PR (same policy, diff, discussion, and model), so that report is reposted without a new model call.}

_{View job · claude-sonnet-5-5 (high) · reused (no model call)}