Warn for string-to-literal narrowing assignments - #19603

Description

Allows assignments from plain string values to fixed string-literal receivers, such as 'foo' | 'bar', to compile with a warning when the value may be valid at runtime.

The assignment is treated as narrowed to the receiver’s allowed string literals, matching existing narrowing behavior for bounded values.

Fixes #11889

Example Usage

// mod.bicep
@allowed(['foo', 'bar'])
param foobar string

// main.bicep
param foobar string
module mod './mod.bicep' = {
  name: 'mod'
  params: {
    foobar: foobar
  }
}

This now emits warning-level BCP036 instead of error-level BCP036

Checklist

• I have read and adhere to the contribution guide.

Microsoft Reviewers: Open in CodeFlow