Remove legacy type checking code and update documentation

[yuancu]

Summary

Sub-PR 4/4 for #4892 — splitting the large PR into reviewable pieces targeting feature/validation.

This PR removes legacy type checking code that is no longer needed after migrating to Calcite's validation system, and updates user-facing documentation.

Deletions

• PPLTypeChecker.java (701 lines) — Legacy type checker interface, replaced by Calcite's operand type checking
• CalciteFuncSignature.java — Unused after PPLFuncImpTable simplification in Sub-PR B
• CoercionUtils.java (old location) — Functionality moved to calcite/validate/ package in Sub-PR A
• OpenSearchSparkSqlDialect.java (old ppl/ location) — Moved to core/calcite/validate/ in Sub-PR A
• LambdaUtils.java — Lambda inference methods consolidated into TransformFunctionImpl/ReduceFunctionImpl

Documentation Updates

• docs/dev/ppl-functions.md
• docs/user/ppl/cmd/bin.md
• docs/user/ppl/cmd/patterns.md
• docs/user/ppl/functions/collection.md
• docs/user/ppl/functions/conversion.md
• docs/user/ppl/interfaces/endpoint.md

Dependency

• Requires Enable validation pipeline and update all test expected outputs #5215 (Sub-PR C) merged first
• Merge after rebasing onto updated feature/validation

Signed-off-by: Yuanchun Shen yuanchu@amazon.com

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