Fix Collection builder example to be well-behaved with proper buffer allocation

[Copilot]

The LineBuffer class in the Collection builder example was not well-behaved according to the documentation's own definition. It had two main issues:

1. Enumeration mismatch: When creating a collection with a few elements, it would enumerate all 80 elements from its internal buffer, violating the principle that "The value of Count or Length on a countable collection produces the same value as the number of elements when enumerated."

2. Memory waste: It always allocated a fixed 80-character buffer regardless of actual usage, wasting memory for smaller collections.

Before this fix:

LineBuffer line = ['H', 'e', 'l', 'l', 'o'];  // 5 elements
// - Allocates 80 characters (wastes 75)
// - foreach would enumerate 80 elements instead of 5
// - No Count property available

After this fix:

LineBuffer line = ['H', 'e', 'l', 'l', 'o'];  // 5 elements
Console.WriteLine(line.Count);                 // 5
// - Allocates exactly 5 characters
// - foreach enumerates exactly 5 elements
// - Proper indexer access with bounds checking

Changes made:

• Added _count field to track the actual number of elements set in the constructor
• Changed buffer allocation from fixed new char[80] to dynamic new char[buffer.Length]
• Added Count property to make the collection countable
• Modified GetEnumerator() to only yield actual elements using yield return
• Added indexer this[int index] with proper bounds checking for direct element access
• Fixed IEnumerable.GetEnumerator() to delegate to the generic version

The collection now properly demonstrates the "well-behaved collection" concept with efficient memory usage and correct enumeration behavior.

Fixes #41756.

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[BillWagner]

Good start, but the code needs more updates.

[BillWagner]

@copilot Can you address the remaining comment?

[Copilot]

@copilot Can you address the remaining comment?

Fixed the buffer allocation issue. The LineBuffer now allocates exactly the buffer size needed instead of a fixed 80-character array. Changes made in commit 425a8b2:

• Removed the fixed allocation = new char[80]
• Updated constructor to allocate _buffer = new char[buffer.Length]
• Simplified the logic to copy all elements since buffer sizes now match

The collection now properly allocates only the memory it needs while maintaining the well-behaved collection behavior.

[BillWagner]

This is now ready for final review