RBF Coop Close using sat/kw instead of sat/vb

[MPins]

Changed the RBF Coop Close to make it use internally sat/kw instead of sat/vb.

This PR must be merged before PR#10067 as it provides the necessary groundwork.

[gemini-code-assist]

Summary of Changes

Hello @MPins, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refactors the RBF cooperative close process to align all fee rate calculations and representations with the sat/kw unit. This change standardizes the internal handling of transaction fees, improving consistency and preparing the codebase for upcoming features that rely on this unified approach. The modifications span across API definitions, internal state management, and fee estimation logic.

Highlights

• Fee Rate Unit Standardization: The RBF cooperative close mechanism has been updated to consistently use sat/kw (satoshi per kiloweight) instead of sat/vb (satoshi per virtual byte) for all internal fee rate calculations and representations.
• API and Data Structure Updates: The lnrpc/lightning.proto and lnrpc/lightning.swagger.json files have been modified to include a new fee_per_kw field in the PendingUpdate message, reflecting the change in fee rate units for external communication.
• Internal Logic Refinement: Core logic within lnwallet/chancloser and peer packages has been adjusted to handle chainfee.SatPerKWeight types directly, removing previous conversions and ensuring consistent fee handling throughout the RBF cooperative close state machine.
• Preparation for Future PR: This pull request serves as foundational work, providing the necessary changes for a subsequent PR (Fees: add fractional sat/vB support (lncli) and sats_per_kw (RPC) #10067) that will likely build upon this standardized fee rate handling.

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[gemini-code-assist]

Code Review

This pull request refactors the RBF cooperative close logic to use sat/kw instead of sat/vb for fee rates. The changes are consistent across the codebase, including protocol definitions, RPC server logic, and tests. My feedback includes a suggestion to deprecate the old fee_per_vbyte field in the protobuf definition for API clarity and an improvement to a comment to better align with the style guide by explaining the intention of the code.