Fix C# back-compat service method overloads for reserved param names; suppress AZC0002 when previous method ends in CancellationToken

[Copilot]

Back-compat overloads generated for service methods (added in 72a95c2) produced uncompilable C# when previous parameters had raw input names that are not valid C# identifiers (e.g. OData query parameters such as $select, $top, $skip, $count). They also could not be emitted correctly when the previous method ended in a CancellationToken parameter — making CT optional introduces a CS0121 ambiguity at basic call sites with the new method, and making CT required violates the SDK guideline that CancellationToken parameters must be optional.

Changes

• ClientProvider.BuildBackCompatOverloadForNewOptionalParameters — emit named-argument labels using the C# variable name (select:, top:) rather than the raw input name ($select:, $top:). Match previous→current parameters by ToVariableName() so renames that differ only by reserved characters still align.
• ClientProvider.ProcessBackCompatForNewOptionalParameters — when the previous method's trailing parameter is a CancellationToken, the back-compat overload is still emitted (with the trailing CT required, to avoid a CS0121 ambiguity with the new method) but is wrapped in #pragma warning disable AZC0002 / #pragma warning restore AZC0002 so the SDK analyzer rule that requires CancellationToken parameters to be optional does not fire on the hidden compatibility overload. The trailing-parameter type comparison uses CSharpType.CSharpTypeIgnoreNullableComparer so nullability is ignored.
• Tests — the four existing back-compat baselines now include the back-compat overloads with the AZC0002 pragma wrappers around them. Added a new BackCompatibility_NewOptionalParameterWithReservedName test (with testdata files) that exercises the reserved-name fix via a previous-contract protocol method (ending in RequestOptions, so no AZC0002 suppression is required) plus a new $select query parameter, verifying the generated back-compat overload uses @select: default rather than the buggy $select: default.

Example (reserved-name fix)

Before:

[EditorBrowsable(EditorBrowsableState.Never)]
public virtual AsyncPageable<SearchIndexResponse> GetIndexesWithSelectedPropertiesAsync(
    IEnumerable<string> @select, CancellationToken cancellationToken)
{
    return this.GetIndexesWithSelectedPropertiesAsync(
        $select: default, $top: default, $skip: default, $count: default,
        cancellationToken: cancellationToken);
}

After:

#pragma warning disable AZC0002 // Back-compat overload preserves the previous method signature where CancellationToken was the trailing parameter. Making it optional would introduce an ambiguous call with the new method.
[EditorBrowsable(EditorBrowsableState.Never)]
public virtual AsyncPageable<SearchIndexResponse> GetIndexesWithSelectedPropertiesAsync(
    IEnumerable<string> @select, CancellationToken cancellationToken)
{
    return this.GetIndexesWithSelectedPropertiesAsync(
        select: default, top: default, skip: default, count: default,
        cancellationToken: cancellationToken);
}
#pragma warning restore AZC0002 // ...

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commit: e89375b

[jorgerangel-msft]

Can we also please get a regen preview? It might also be worth running the debug profile script against Azure/azure-sdk-for-net#59156 to verify the changes