Use V prefix for digit-starting members in API version enums

[Copilot]

Enum members in Versions enums that start with digits were being prefixed with the full enum type name (e.g., Versions20250101) instead of the conventional V prefix (e.g., V20250101) that teams were already manually applying via @@clientName.

Changes

• emitModels.ts: When a digit-starting enum member gets its _ prefix replaced, check if the enum has UsageFlags.ApiVersionEnum. If so, use "V" instead of the full enum type name as the prefix.
• New test scenario apiVersionWithDigitStartingMembers.md: Validates V prefix for backtick-quoted date-style version members.
• Updated tests extends.md, enumKeyNorm.md: Reflect the new V prefix behavior for existing API version enum fixtures.

Before / After

// TypeSpec
enum Versions {
  `2025-01-01`: "2025-01-01",
}

// Before
export enum KnownVersions {
  Versions20250101 = "2025-01-01",
}

// After
export enum KnownVersions {
  V20250101 = "2025-01-01",
}

[kazrael2119]

@copilot fix smoke test ci failure

[Copilot]

@copilot fix smoke test ci failure

Fixed in commit 38fd45d. The smoke test was failing because 4 generated files in packages/typespec-test/test/ (batch_modular, ai, overloads_modular, widget_dpg) still had the old Versions prefix. I regenerated them with the new V prefix and the smoke test now passes locally ("All specs succeeded!").