Fix stale TODO in BufferedFileStreamStrategy

[stephentoub]

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[Copilot]

Pull Request Overview

This PR inlines the AllocateBuffer helper from a private method into a local function within EnsureBufferAllocated, removes a stale TODO, and updates the buffer allocation call.

• Inlined AllocateBuffer into EnsureBufferAllocated as a local function
• Removed the TODO about converting to a local function
• Updated Interlocked.CompareExchange usage inside the new local function

Comments suppressed due to low confidence (2)

src/libraries/System.Private.CoreLib/src/System/IO/Strategies/BufferedFileStreamStrategy.cs:1030

• The MemberNotNull attribute on the local function does not inform the null-state of the outer method. To ensure the compiler recognizes that _buffer is non-null after calling EnsureBufferAllocated, move this attribute to the EnsureBufferAllocated method signature instead.

                [MemberNotNull(nameof(_buffer))]

src/libraries/System.Private.CoreLib/src/System/IO/Strategies/BufferedFileStreamStrategy.cs:1031

• Local functions do not support inlining attributes in the same way as top-level methods; this annotation may be ignored or cause a compile error. Consider removing it or moving it to the outer EnsureBufferAllocated method if no-inlining is required.

                [MethodImpl(MethodImplOptions.NoInlining)]

[stephentoub]

/ba-g unrelated test failures