feat(compiler): emit missing-extern-declaration diagnostic for orphaned $functions exports

[timotheeguerin]

Summary

Fixes #10748

When a JS file exports $functions without a corresponding extern fn declaration in TypeSpec, calling that function silently produces incorrect results (no error, no parameters).

Changes

• New diagnostic: missing-extern-declaration fires at program check time for ALL orphaned function implementations (regardless of whether the function is called)
• Checker: Added checkOrphanedFunctionImplementations() in checkProgram() that walks JS source file symbols looking for Function symbols without a matching FunctionDeclarationStatementNode
• Tests: Refactored functions.test.ts to use per-function JS file testers (avoiding orphan diagnostics in tests)
• Semantic walker: Updated to use compileAndDiagnose to handle the new diagnostic

Diagnostic message

Function implementation "<name>" is exported in JS via $functions but has no corresponding 'extern fn' declaration in TypeSpec.

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/compiler@10773

commit: 87bee3c

[github-actions]

All changed packages have been documented.

• ✅ @typespec/compiler

Show changes

@typespec/compiler - fix ✏️

Report an error when a function is declared in the $functions map in a JS file but has no corresponding extern fn declaration in TypeSpec. Previously this would silently have no effect.

[azure-sdk-automation]

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