Fix #3894: keep lambda parameter typing consistent

[sailro]

Fixes #3894.

Problem

When a lambda parameter has an anonymous type, ILSpy omits that unnameable type but leaves
sibling parameter types explicit. The resulting mixed form, such as
(item, int context) => ..., is rejected with CS0748.

Any source lambda with an anonymous-typed parameter must have used implicit typing for its
whole parameter list, because the anonymous type cannot be written explicitly.

Solution

Build the parameter list first, detect whether any parameter contains an anonymous type, and
then remove every parameter type when the remaining parameter syntax permits implicit typing.

This keeps the emitted syntax and DecompiledLambdaResolveResult.IsImplicitlyTyped consistent,
while leaving unrepresentable modifier/attribute combinations as a conservative fallback.

Tests

Added a Roslyn Pretty regression using a generic delegate whose first parameter becomes an
anonymous type and whose second parameter is int.

Validation:

• AnonymousTypes Pretty matrix: 24 passed

• Full Pretty suite: 1,906 passed, 5 existing skips

• Release solution build: 0 warnings, 0 errors

• Full solution test suite: 4,885 passed, 15 existing skips

• At least one test covering the code changed