Fix SRTP get_Item witness resolution for string indexers

[Copilot]

SRTP calls constrained to member get_Item could hit an internal compiler error on string (unknown builtin witness 'get_ItemDynamic') instead of resolving to the string indexer. This change aligns string handling with existing builtin witness lowering for indexers.

• Compiler fix: correct builtin witness shape for string get_Item

  • Updated TcGlobals.TryMakeOperatorAsBuiltInWitnessInfo to match the actual compiled argument shape for string indexer access.
  • get_Item on string now resolves through the existing getstring_info path rather than falling through to dynamic builtin witness lookup.

• Regression coverage

  • Added a focused component test in IWSAMsAndSRTPsTests verifying SRTP member get_Item: int -> ^U works against string at compile/run time.

let inline indexInto (slice: ^T when ^T: (member get_Item: int -> ^U)) i : ^U =
    slice.get_Item i

let c = indexInto "abcde" 2   // now resolves correctly to string indexer

[T-Gro]

LGTM — clean, correct fix.

Root cause analysis: The pattern [stringTy; _; _] expected 3 elements in CompiledObjectAndArgumentTypes, but for string.get_Item(int) the actual shape is [string; int] (2 elements — object type + parameter type). This is consistent with the get_Item array case on the line above which uses [arrTy; _]. The mismatch meant the branch never fired, falling through to MakeBuiltInWitnessInfo which has no get_ItemDynamic case, hence the internal error.

Fix correctness: Changing to [stringTy; _] now matches correctly. Passing [] as tyargs is right because getstring_info (IntrinsicFunctions.GetString) is non-generic. The argExprs forwarding (string instance + int index) matches the [[v_string_ty]; [v_int_ty]] signature.

Test: Appropriate runtime test since the bug manifests at codegen/execution, not type-checking. --nowarn:77 correctly suppresses the special-status-member-constraint warning.

[github-actions]

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