Remove deprecated fee rate option --sat_per_byte

[MPins]

As defined at release doc v0.21.0 we now remove the option --sat_per_byte.

Change Description

The sat_per_byte field was deprecated back in v0.13.0 in favor of sat_per_vbyte. This PR completes its removal: the deprecated field is no longer honored by the RPC server, the CLI, or the wallet RPC, and any request that still sets it is now explicitly rejected.

Behavior change

Previously, sat_per_byte was still accepted and silently converted, with a guard preventing both fields from being set at once.

Now, if a request sets sat_per_byte the ErrSatPerByteRemoved is returned.

[github-actions]

🔴 PR Severity: CRITICAL

Highest-severity file determines label | 7 non-test files | ~225 lines changed

🔴 Critical (1 file)

• rpcserver.go - Core server coordination file; listed explicitly as CRITICAL in classification rules

🟠 High (2 files)

• lnrpc/rpc_utils.go - RPC/API utility code
• lnrpc/walletrpc/walletkit_server.go - RPC/API wallet kit server implementation

🟡 Medium (3 files)

• cmd/commands/cmd_open_channel.go - CLI client command (cmd/* -> MEDIUM regardless of filename keywords)
• cmd/commands/commands.go - CLI client commands
• cmd/commands/walletrpc_active.go - CLI client walletrpc command (MEDIUM, not HIGH -- under cmd/*)

🟢 Low / Excluded (4 files)

• docs/release-notes/release-notes-0.22.0.md - Release notes (docs/*)
• itest/lnd_wallet_import_test.go - Integration test (excluded from severity count)
• lnrpc/walletrpc/walletkit_server_test.go - Test file (excluded from severity count)
• rpcserver_test.go - Test file (excluded from severity count)

Analysis

This PR touches rpcserver.go, the core server coordination file, which is explicitly listed as a CRITICAL-tier file requiring expert review. The PR also modifies RPC/API layer files under lnrpc/ (HIGH) and CLI client commands under cmd/ (MEDIUM).

Severity bump rules were checked:

• File count: 7 non-test/non-generated files (threshold: >20) -- no bump
• Lines changed: ~225 (threshold: >500) -- no bump
• Multiple distinct critical packages: Only one critical file (rpcserver.go) -- no bump

The CRITICAL classification stands based on the presence of rpcserver.go alone.

---

_{To override, add a severity-override-{critical,high,medium,low} label.}
<!-- pr-severity-bot -->

[gemini-code-assist]

Summary of Changes

Hello, 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 finalizes the removal of the deprecated sat_per_byte field, which has been superseded by sat_per_vbyte. By enforcing this change at the RPC and CLI layers, the codebase is simplified and technical debt is reduced. The implementation ensures that any legacy usage is now explicitly rejected, maintaining consistency across the API.

Highlights

• Removal of Deprecated Field: The sat_per_byte field, deprecated since v0.13.0, has been completely removed from the CLI, RPC server, and wallet RPC interfaces.
• Explicit Rejection: Requests that still include the sat_per_byte field are now explicitly rejected with the lnrpc.ErrSatPerByteRemoved error.
• Codebase Cleanup: Removed redundant validation logic, simplified helper functions like CalculateFeeRate, and cleaned up CLI flags across multiple commands.
• Test and Documentation Updates: Updated integration and unit tests to use sat_per_vbyte and added a breaking change entry to the release notes.

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[gemini-code-assist]

Code Review

This pull request removes the deprecated --sat_per_byte fee rate option across multiple CLI commands and RPCs (including CloseChannel, OpenChannel, SendCoins, SendMany, and BumpFee) in favor of sat_per_vbyte. It also updates the corresponding tests and release notes. The review feedback identifies a broken markdown link in the release notes that needs to be corrected.

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