Enable plugin evaluation of String API overloads

[koperagen]

This PR enables plugin interpretation for String overloads that have CS DSL counterparts, see
Plugin.kt:

Marks String API overloads (functions with vararg String parameters) with metadata to enable automatic delegation to their typed counterparts implementations in the compiler plugin. This annotation should not have default values for arguments (breaks plugin resolve).
Params:
interpreter - The interpreter name from the corresponding @Interpretable annotation on the typed API function (the one with ColumnSelectionDsl parameter)
targetArgument - The name of the ColumnSelectionDsl parameter in the typed API function

RawSchemaOperationsResultCorrectnessTest:

Making sure compiler evaluation of String API overload on raw dataframe (no schema) is correct whatever columns appear as a result are "type safe"; 
no ColumnNotFound or ClassCast exceptions will happen if property is used. 

User scenario when df has no schema, some string operations are invoked, and as a result we enrich schema

TypedSchemaOperationsResultConsistencyTest:

Making sure compiler evaluates String API overloads same as CS DSL overloads on typed input.
Sometimes some precision is expected to be lost, see Merge0

User scenario when dataframe itself is typed, but user chose to use String overload for simplicity => we do not loose information

I had to update kotlin to 2.4.20-RC2 so that compiler plugin understands StringApiInterpretable, to have these tests in this repo, i think it's better for visibility in this case

I plan to update this PR: clean up lint.kt and add annotations to a few remaining functions.
Check the overall idea, clarity of tests, etc.

[jetbrains-air]

Really like where this is heading — two things to sort out before merge, then this is good to go.

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[jetbrains-air]

Blocking: unused import.

Problem: StringApiInterpretable is imported here and in parse.kt:9, but neither file ever uses it — consistent with lint.kt, which lists Convert0 and Parse as "own interpreter". These are leftovers from the annotation sweep.

Failure scenario: standard:no-unused-imports is active (ktlint_code_style = ktlint_official, and the rule is not among the ktlint_standard_* opt-outs in .editorconfig; core/src/main is not in the ktlint = disabled glob). ./gradlew build fails :core:ktlintMainSourceSetCheck on both files, so CI goes red.

Suggested fix: delete the import line here and at parse.kt:9.

[jetbrains-air]

Thanks for this — requesting changes, since there are blocking findings inline to address before merge.

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[jetbrains-air]

Blocking: bare @Refine is added here and on ~30 other String overloads whose only interpreter metadata is @StringApiInterpretable — an annotation only the 2.4.20+ plugin understands. Compiler-Plugin.md documents 2.2.20 as the minimum supported version, and this is the first @Refine in the tree without a co-located @Interpretable.

Failure scenario: a user on Kotlin 2.2.20–2.4.0 upgrades DataFrame. FunctionCallTransformer.intercept gates only on the Refine annotation, so the call is intercepted and its return type is replaced by a generated token; analyzeRefinedCallShape then finds no @Interpretable, falls back to PluginDataFrameSchema.EMPTY, and df.groupBy("key").toDataFrame().key no longer resolves — code that compiles today stops compiling.

Fix: don't emit bare @Refine; have the new plugin trigger refinement from @StringApiInterpretable itself (or from a distinctly named annotation older plugins ignore) so pre-2.4.20 behaviour is unchanged. If you'd rather keep @Refine, raise the documented minimum plugin version and call the break out in the release notes.

[jetbrains-air]

Blocking: this overload already has a purpose-built @Interpretable("SelectString"). It is the only declaration in the tree carrying both annotations, and precedence between them isn't defined anywhere.

Failure scenario: if the plugin reads @StringApiInterpretable first, the most-used String overload silently switches from SelectString to the generic Select0 delegation and SelectString becomes dead code. Neither new test suite can catch it, because both paths are asserted to agree.

Fix: remove @StringApiInterpretable here and add Select0 to the "own interpreter" ignore list in lint.kt, next to Parse and Convert0.