Replace CancellationToken overloads with a single optional parameter

[Copilot]

The implementation generated two overloads per operation (with/without CancellationToken) while the intended API surface was a single method with an optional cancellationToken parameter defaulting to CancellationToken.None.

Changes

• Optional parameter instead of overloads: Each generated method now has a single optional cancellationToken: CancellationToken parameter. Passing null as optionalValue to ProvidedParameter correctly encodes default(CancellationToken) in CLI metadata — identical to how C# encodes ct = default for struct parameters.
• Simplified compileOperation: Removed includeCancellationToken: bool flag; CT parameter is always appended unconditionally.
• Simplified invokeCode: CT is always extracted from the last argument position — no conditional branch.
• Reduced generated surface area: N methods instead of 2N per client type.

// Before: two overloads generated per operation
[ compileOperation uniqueName op false; compileOperation uniqueName op true ]

// After: single method with optional CT parameter
[ compileOperation uniqueName op ]
// Parameter: ProvidedParameter("cancellationToken", typeof<CancellationToken>, optionalValue = (null: obj))

Callers with no CT argument continue to work unchanged; callers that want explicit cancellation pass the token directly.

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