Fix formatter regression for union as named template argument

[timotheeguerin]

Fixes #11092

Problem

PR #11010 fixed a blank-line / over-indent issue when a union expression is used as one of several positional template arguments. That fix made printUnion suppress its own leading line + indent when the union is a direct argument of a multi-argument template reference (the surrounding argument list already supplies them).

This regressed the named template argument case (Name = <union>). For a named argument the union is printed after Name = , so the argument list's line break / indent does not apply to the union variants. Suppressing the union's own break + indent collapsed the variants:

alias Alias = Test<
  A = "Some long value",
  TakesALongUnion = | string
  | int32
  | int64
  | "Some very long string that split line"
>;

Fix

isInMultiTemplateArgumentList now returns false when the parent TemplateArgument has a name. Named-argument unions fall back to the standalone behavior (their own leading line + | + indent):

alias Alias = Test<
  A = "Some long value",
  TakesALongUnion =
    | string
    | int32
    | int64
    | "Some very long string that split line"
>;

The single-argument case and the positional multi-argument case (#11009) are unchanged.

Tests

• Added a regression test for the named-union multi-argument case (Regression in formatting #11092).
• Existing Formatter adds a blank line and over-indents a union used as a template argument #11009 positional tests still pass; full formatter suite (216 tests) passes.
• prettier --check "**/*.tsp" clean; compiler builds.

[pkg-pr-new]

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npm i https://pkg.pr.new/@typespec/compiler@11093

commit: 555ec97

[github-actions]

All changed packages have been documented.

• ✅ @typespec/compiler

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@typespec/compiler - internal ✏️

Fix formatter regression where a union used as a named template argument lost its leading | indentation

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