refactor: add ColumnFormat implementation for string columns

[jaykanakiya]

Description

Addresses a follow up from #19258

Adds StringDictionaryEncodedColumnFormat that persists StringColumnFormatSpec in DictionaryEncodedColumnPartSerde segment metadata.

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Key changed/added classes in this PR

• StringDictionaryEncodedColumnFormat
• DictionaryEncodedColumnPartSerde
• StringDimensionIndexer
• StringDimensionHandler

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This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.

[FrankChen021]

Severity	Findings
P0	0
P1	2
P2	0
P3	0
Total	2

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This is an automated review by Codex GPT-5

[FrankChen021]

[P1] Persisted string format drops multi-value handling when rebuilding handlers

getColumnHandler reconstructs the StringDimensionHandler with MultiValueHandling.ofDefault() even when columnFormatSpec contains an explicit multiValueHandling. IndexMergerBase and IncrementalIndex.loadDimensionIterable call getColumnHandler directly, so compacting or loading persisted string columns with ARRAY or SORTED_SET can silently switch back to the default behavior and reorder or deduplicate multi-value strings. Derive the handler's multiValueHandling from columnFormatSpec when present, matching StringDimensionSchema.getDimensionHandler.

[FrankChen021]

[P1] String format merge bypasses type compatibility checks

This delegation happens before CapabilitiesBasedFormat's existing type checks, and StringDictionaryEncodedColumnFormat.merge accepts any CapabilitiesBasedFormat without validating its logical type. If one segment has a string-specific format for a column and another legacy segment reports the same column as LONG, DOUBLE, COMPLEX, etc., the merge now produces a STRING format instead of rejecting the incompatible schemas. Restrict delegation to compatible string capabilities or perform the same logical/element/complex type validation in StringDictionaryEncodedColumnFormat.merge before merging.