Add inputlookup as another form of search sources

[Swiddis]

Description

Allows queries like inputlookup big5 | head 1.

Will rip this out again later once #5074 is actually in progress and replace it with a real inputlookup command, this is just some temp noise since we want to call plain indices lookups in the current version.

Related Issues

N/A

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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[coderabbitai]

📝 Walkthrough

Summary by CodeRabbit

• New Features
  • Added INPUTLOOKUP as a new data-source option for queries. It can be referenced as a table, invoked as a table function, or used as a dynamic source in FROM clauses, giving more flexible lookup-table access and retrieval.

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Walkthrough

Added an INPUTLOOKUP lexer token and updated the fromClause grammar to allow INPUTLOOKUP as a source via new parser alternatives (INPUTLOOKUP tableOrSubqueryClause, INPUTLOOKUP tableFunction).

Changes

Cohort / File(s)	Summary
ANTLR Grammar Lexer Token ppl/src/main/antlr/OpenSearchPPLLexer.g4	Added INPUTLOOKUP: 'INPUTLOOKUP'; under dataset types to recognize the INPUTLOOKUP keyword.
ANTLR Grammar Parser Rules ppl/src/main/antlr/OpenSearchPPLParser.g4	Extended fromClause with INPUTLOOKUP alternatives: INPUTLOOKUP tableOrSubqueryClause and INPUTLOOKUP tableFunction, enabling INPUTLOOKUP as a data source option.

Sequence Diagram(s)

(omitted — change is a small grammar addition that doesn't introduce multi-component control flow)

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Suggested reviewers

• penghuo
• ps48
• kavithacm
• derek-ho
• joshuali925
• dai-chen
• LantaoJin
• anirudha
• ykmr1224
• qianheng-aws

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: adding inputlookup as a search source option in the grammar files.
Description check	✅ Passed	The description is directly related to the changeset, explaining the purpose of the inputlookup addition and its temporary nature pending issue #5074.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@ppl/src/main/antlr/OpenSearchPPLParser.g4`:
- Around line 623-632: The parser allows INPUTLOOKUP in the fromClause with the
dynamicSourceClause alternative, but visitDynamicSourceClause currently throws
UnsupportedOperationException causing a runtime crash for INPUTLOOKUP
dynamicSourceClause; implement visitDynamicSourceClause (or the corresponding
visitor method) to properly handle dynamicSourceClause when its parent is
INPUTLOOKUP by wiring the same dispatch/processing done for
tableOrSubqueryClause/tableFunction (or return a clear
UnsupportedOperationException only for truly unsupported subvariants), ensuring
the visitor inspects the parse node shape and delegates to the correct
downstream handler; refer to visitDynamicSourceClause, fromClause, INPUTLOOKUP,
and dynamicSourceClause to locate and fix the logic.

[dai-chen]

Could you link an issue to provide more context about the plan?

[dai-chen]

Any syntax or AST test to add? No other code changes required?

[ykmr1224]

I am not sure if this change is actually needed.
I think inputlookup is needed if we cannot directly query lookup tables with source=xxxx syntax.