Anonymize xyseries command and mark it experimental in docs

[penghuo]

Description

Follow-up to #5343 (xyseries command).

• Adds visitXyseries to PPLQueryDataAnonymizer so the xyseries stage appears in the anonymized query logged for every PPL request, with pivot literals and options masked. Previously the visitor fell through to visitChildren and the entire | xyseries ... in ("F","M") ... clause was silently dropped from log output (including raw pivot-value literals).
• Marks xyseries as experimental (since 3.8) in docs/user/ppl/index.md, matching how other newly-introduced commands are listed.

Related Issues

Follow-up to #5343

Check List

• New functionality includes testing.
• New functionality has been documented.
• New functionality has javadoc added.
• New functionality has a user manual doc added.
• New PPL command checklist all confirmed.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff or -s.
• Public documentation issue/PR created.

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[github-actions]

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🧪 PR contains tests

🔒 No security concerns identified

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⚡ Recommended focus areas for review Possible Issue The anonymizer always outputs 'in (***)' even when the xyseries command has no pivot values specified. If the xyseries syntax allows omitting the 'in' clause or having an empty list, this will produce incorrect anonymized output that doesn't match the original query structure. command.append(" in (").append(MASK_LITERAL).append(")");

[github-actions]

PR Code Suggestions ✨

Explore these optional code suggestions:

Category	Suggestion	Impact
General	Define magic string as constant The hardcoded separator check ": " appears to be a magic value without context. If this is the default separator value, consider defining it as a constant for better maintainability and clarity. This makes the code more self-documenting and easier to update if the default changes. ppl/src/main/java/org/opensearch/sql/ppl/utils/PPLQueryDataAnonymizer.java [833-835] -if (node.getSeparator() != null && !": ".equals(node.getSeparator())) { +private static final String DEFAULT_SEPARATOR = ": "; +... +if (node.getSeparator() != null && !DEFAULT_SEPARATOR.equals(node.getSeparator())) { command.append(" sep=").append(MASK_LITERAL); } Suggestion importance[1-10]: 4 __ Why: While defining ": " as a constant would improve code maintainability and readability, this is a minor improvement that doesn't address any functional issue or bug. The magic string is only used once in this context.	Low