Fix ICE on named-arg indexer setter (#16034)

[T-Gro]

Summary

Fixes #16034 — calling an indexed property setter with a named argument (e.g. .indexed1(a1="ok") <- 1) caused an internal compiler error (FS0193 / ArgumentException: The input must be non-negative).

Root Cause

When a named argument matches one of the indexer arguments of a property setter, the remaining unnamed called args may number fewer than 2. The ParamArray detection in MethodCalls.fs used
UnnamedCalledArgs - 2 unconditionally for indexer setters, leading to a negative index into calledArgs.

Fix

• Gate the setter-shape indexing on
  UnnamedCalledArgs >= 2 and fall back to the regular path otherwise.
• Skip the unnamed-arg-prefix deprecation check for indexer setters, since the trailing 'value' arg breaks the strict (i, j) position match when an indexer arg is supplied by name.
• Fix a minor issue in ConstraintSolver.fs where a property-setter indexer arg count was off-by-one.

Tests

Added IndexedSetterNamedArgTests.fs covering:

• Single-index setter with named arg
• Multi-index setter with one named arg
• Named �alue arg on setter
• Extension-method indexed setter with named arg

All variants compile and typecheck without error.

[github-actions]

❗ Release notes required

@T-Gro,

Caution

No release notes found for the changed paths (see table below).

Please make sure to add an entry with an informative description of the change as well as link to this pull request, issue and language suggestion if applicable. Release notes for this repository are based on Keep A Changelog format.

The following format is recommended for this repository:

* <Informative description>. ([PR #XXXXX](https://github.com/dotnet/fsharp/pull/XXXXX))

See examples in the files, listed in the table below or in th full documentation at https://fsharp.github.io/fsharp-compiler-docs/release-notes/About.html.

If you believe that release notes are not necessary for this PR, please add NO_RELEASE_NOTES label to the pull request.

You can open this PR in browser to add release notes: open in github.dev

Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/11.0.100.md	No release notes found or release notes format is not correct