fix(repeating_group): add Reset() method (closes #758)

[caiyi0616]

Summary

Add RepeatingGroup.Reset() method to clear all groups from a RepeatingGroup instance, allowing reuse without reallocating the underlying template and struct. This addresses issue #758.

Motivation

In high-throughput FIX message processing scenarios, each incoming message may contain a repeating group with a variable number of entries. Previously, parsing required allocating a new RepeatingGroup instance for each parse, causing unnecessary allocations and GC pressure. With Reset(), callers can reuse the same instance across multiple message parses, reducing allocations in hot paths.

Changes

repeating_group.go

// Reset clears all groups from this RepeatingGroup, allowing the instance to be
// reused for a new message parse without reallocating the template and struct.
func (f *RepeatingGroup) Reset() {
    f.groups = f.groups[:0]
}

repeating_group_test.go

Added comprehensive tests:

• TestRepeatingGroup_Reset: Verifies that Reset() clears all groups and keeps capacity
• TestRepeatingGroup_ResetMultipleCycles: Verifies instance reuse across multiple parse cycles without reallocation

Verification

• All existing tests pass (go test ./...)
• New tests: go test -run "Reset" -v
• Memory allocation profile confirms reduced allocations in repeated-parse scenarios

Closes #758