Add chunked header indicator to AofHeader

[TedHartMS]

Also change 'padding' to 'flags' with reorganization of header bits for clarity

[Copilot]

Pull request overview

This PR updates the AOF record header encoding to make the “type” and other per-record indicators explicit: it renames the byte at offset 1 from padding to flags, packs AofHeaderType into masked bits, and adds flag bits intended for chunked records and FLUSH “unsafe truncate” behavior.

Changes:

• Replaced padding usage with a flags bitfield and added AofHeader.HeaderType accessor for masked header-type encoding.
• Replaced raw unsafeTruncateLog byte overlay with an UnsafeTruncateLog boolean accessor backed by a flag bit.
• Updated AOF enqueue/replay codepaths (and the offline benchmark generator) to use the new accessors.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
libs/server/AOF/GarnetLog.cs	Updates AOF record enqueue paths to set HeaderType and UnsafeTruncateLog via the new header accessors.
libs/server/AOF/AofProcessor.cs	Updates replay logic to read HeaderType/UnsafeTruncateLog from flags rather than interpreting the old overlayed bytes.
libs/server/AOF/AofHeader.cs	Introduces flags bitfield, masks for header type, and boolean/accessor properties for header type and unsafe-truncate.
benchmark/Resp.benchmark/OfflineBench/AOFBench/AofGen.cs	Adjusts benchmark AOF generation to use the new HeaderType accessor.