[Repo Assist] Add tests for begin..end expressions and auto-property edge cases

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Summary

Adds 6 new tests to two under-covered test files to improve test suite coverage and document expected formatting behaviour.

BeginEndTests.fs (was 2 tests → now 6)

Test	Scenario
begin end in value binding	begin..end in a let-binding value
begin end in function body	begin..end as a function body result
begin end in match branch	begin..end in each arm of a match expression
begin end with comment inside	begin..end block containing a line comment

AutoPropertiesTests.fs (was 4 tests → now 6)

Test	Scenario
auto-property with attribute	[(DefaultValue)] attribute on the preceding line of a member val
static auto-property with explicit type	static member val with an explicit type annotation

Rationale

Both files had very few tests (2 and 4 respectively). These additions document common F# constructs that were not explicitly covered and act as regression guards for those formatting paths.

Test Status

• ✅ Build: succeeded (0 warnings, 0 errors)
• ✅ Tests: 2745 passed, 7 skipped, 0 failed
• ✅ All 6 new tests pass
• ✅ Source files already formatted correctly (no fantomas changes needed)

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