refactor(isthmus): remove dead WindowRelFunctionConverter and de-duplicate call converters

[nielspardon]

What

P0 (debris cleanup) of the Isthmus function-mapping refactor — behavior-preserving, no
functional change:

1. Delete the dead WindowRelFunctionConverter. It was never instantiated or wired into
   any ConverterProvider, and it duplicated WindowFunctionConverter. The
   ConsistentPartitionWindow.WindowRelFunctionInvocation capability it targeted is produced
   only in :core (proto read / copy-on-write rewrite) and the :spark integration (its own
   converter) — both untouched — so no reachable capability is removed.
2. De-duplicate the call-converter list. ConverterProvider.getCallConverters() now builds
   on CallConverters.defaults(typeConverter) (previously it re-listed the identical first
   eight converters) and then appends CREATE_SEARCH_CONV and the scalar function converter,
   as before. Order, instances, and the mutable-ArrayList return type are unchanged, so
   subclasses that mutate super.getCallConverters() (e.g. DynamicConverterProvider) keep
   working.
3. Fix stale javadoc in SimpleExtensionToSqlOperator: it already handles window functions
   via toWindowSqlFunction, but the class and three ExtensionCollection overloads still said
   "scalar and aggregate … window functions are not yet implemented".

Why

First, lowest-risk step of the larger function-mapping redesign. See the plan in #1012 and the
umbrella epic #1013.

Verification

• ./gradlew :isthmus:test :isthmus:pmdMain :isthmus:spotlessCheck :isthmus:javadoc — all pass.
• Cross-module compile: :isthmus, :isthmus-cli, :examples:isthmus-api,
  :examples:substrait-spark all compile.
• Behavior-equivalence of the call-converter change was reviewed against CallConverters.defaults()
  and the DynamicConverterProvider override.

Part of #1012 (P0); part of epic #1013.

🤖 Generated with AI

[alexandrefimov]

Checked the mechanical parts against main and they hold up:

• WindowRelFunctionConverter has no references anywhere in the tree except one line in CHANGELOG.md.
• CallConverters.defaults(typeConverter) returns the same eight converters in the same order as the inlined list, the result stays a mutable ArrayList, and DynamicConverterProvider.getCallConverters() still appends to super.getCallConverters() unaffected.
• The SimpleExtensionToSqlOperator javadoc fix is correct — from(...) flat-maps collection.windowFunctions() and toSqlFunction dispatches to toWindowSqlFunction, so the "not yet implemented" note was stale.

Two things worth settling before this lands.

Should this be refactor(isthmus)!? WindowRelFunctionConverter is public, with public constructors and a public convert(...), in the published :isthmus artifact. It is unused in-tree, but it is also self-contained and does something useful on its own, so a downstream that instantiated it directly stops compiling. Comparable isthmus removals were marked breaking (#998, #1035); the title here carries no ! and the description no BREAKING CHANGE: footer, so semantic-release would cut this without flagging the removal.

Worth naming what is being dropped. This is not only dead code — it is the expression half of a Calcite LogicalWindow → Substrait ConsistentPartitionWindow path whose rel half was never written (ConsistentPartitionWindow appears in isthmus only in SqlKindFromRel, i.e. the other direction). Removing it is reasonable and git history keeps it recoverable, but a line in the description saying so would save the next person from rediscovering the gap.

Also needs a rebase — ConverterProvider has moved since mid-July (builder, casing, type observer), and the PR currently shows conflicts.