Restore named timer BatchIf overload compatibility

[dwcullop]

Summary

Restores source compatibility for BatchIf calls that pass a named timer argument (with an optional scheduler) but omit initialPauseState. A compatibility overload is added so those call sites bind again; the batch starts unpaused, matching the previous behavior.

Also documents the overload's initial pause state in the cache operator instructions.

Validation

Prior validation already passed: the solution built across all seven target frameworks, and the focused BatchIf test suite passed. New API surface is captured in the approved API verification files for .NET 8 and .NET 9.

Fixes #1175

[dwcullop]

P2: The new timer overload breaks an existing positional call.

Location: ObservableCacheEx.BatchIf.cs, lines 47-49.

source.BatchIf(pause, null, scheduler) compiles against the base commit, selecting the overload with a nullable timeout. Against this PR it fails with CS0121, because the new IObservable<Unit>? parameter and the existing TimeSpan? parameter both accept the positional null.

Suggested fix: change the added overload shape so named timer calls remain available without introducing another candidate for the existing positional nullable-timeout call. This requires an API-shape decision rather than a null check in the implementation, since resolution fails before any method runs. Include compile-time compatibility coverage for both named-timer calls and the existing BatchIf(pause, null, scheduler) form. Requiring downstream callers to add a cast would leave this as a source-breaking change rather than a compatibility restoration.

[JakenVeina]

Based on the rest of the codebase, we generally don't test operator overloads that are just a no-logic alias of another. I don't see why we should start here.