fix: remove redundant refetchFn() call and fragile BehaviorSubject handling

[michaelbe812]

Summary

• Eliminate redundant double-call to strategy.refetchFn() in mergeRefetchStrategies function
• Remove fragile instanceof BehaviorSubject check that doesn't work reliably with piped observables
• Simplify implementation by letting individual strategies handle their own skip(1) logic

Context

This PR addresses issue #48 which identified performance and logic issues in the mergeRefetchStrategies function.

Problems Fixed

1. Redundant Function Calls: The function was calling strategy.refetchFn() twice - once for the pipe operation and once for the BehaviorSubject check. This was wasteful and could cause side effects with cold streams.

2. Fragile BehaviorSubject Detection: The instanceof BehaviorSubject check was unreliable and created unnecessary coupling between the merge logic and individual strategies.

Changes Made

• Modified mergeRefetchStrategies to call refetchFn() only once per strategy
• Removed the BehaviorSubject instanceof check and associated skip logic
• Simplified the function to filter null/undefined strategies first, then map to observables
• Added comprehensive tests to verify single-call behavior and proper BehaviorSubject handling

Testing

• All existing tests pass ✅
• Added new test to verify refetchFn() is called exactly once per strategy
• Added test to confirm BehaviorSubject initial values are still skipped correctly (handled by the strategy itself)
• Full test suite passes with no regressions

Benefits

• Better performance (no redundant function calls)
• More reliable logic (no fragile instanceof checks)
• Cleaner separation of concerns between merge logic and individual strategies
• Simpler, more maintainable code

Fixes #48