fix(consensus): prevent UB from untrusted bytes interpreted as enum discriminants in consensus headers

[krishvishal]

Closes #2878

Rationale

Consensus headers previously stored Command2 and Operation enum values directly in repr(C) structs. This is unsound for consensus header structs: arbitrary bytes from the network are reinterpreted as enum discriminants without validation, which is undefined behavior in Rust.

What changed?

• Change command and operation fields in all consensus headers from enum types (Command2, Operation) to raw u8 to prevent UB when interpreting untrusted bytes from the network as Rust enum discriminants.
• Add TryFrom<u8> impls for Command2 and Operation with proper error variants (InvalidCommandByte, InvalidOperationByte) in ConsensusError.
• Fix dispatch_request to return Result<Receiver<R>, ConsensusError> instead of silently dropping unrecognized messages.

[codecov]

Codecov Report

❌ Patch coverage is 62.02532% with 90 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.36%. Comparing base (33bee3d) to head (5c632a5).

Files with missing lines	Patch %	Lines
core/common/src/types/consensus/header.rs	74.39%	37 Missing and 5 partials ⚠️
core/common/src/types/consensus/message.rs	35.48%	20 Missing ⚠️
core/shard/src/router.rs	0.00%	14 Missing ⚠️
core/simulator/src/client.rs	0.00%	4 Missing ⚠️
core/consensus/src/impls.rs	0.00%	3 Missing ⚠️
core/consensus/src/plane_helpers.rs	75.00%	3 Missing ⚠️
core/partitions/src/iggy_partitions.rs	0.00%	2 Missing ⚠️
core/metadata/src/impls/metadata.rs	0.00%	1 Missing ⚠️
core/simulator/src/packet.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2887      +/-   ##
============================================
+ Coverage     68.33%   68.36%   +0.02%     
  Complexity      739      739              
============================================
  Files          1053     1053              
  Lines         84763    84916     +153     
  Branches      61297    61460     +163     
============================================
+ Hits          57923    58052     +129     
- Misses        24468    24482      +14     
- Partials       2372     2382      +10     

Flag	Coverage Δ
csharp	67.43% <ø> (-0.19%)	⬇️
go	6.27% <ø> (ø)
java	54.83% <ø> (ø)
node	92.26% <ø> (-0.04%)	⬇️
python	81.57% <ø> (ø)
rust	70.07% <62.02%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
core/consensus/src/namespaced_pipeline.rs	97.20% <100.00%> (ø)
core/metadata/src/stm/mod.rs	46.75% <100.00%> (ø)
core/metadata/src/impls/metadata.rs	16.03% <0.00%> (ø)
core/simulator/src/packet.rs	78.97% <75.00%> (-0.16%)	⬇️
core/partitions/src/iggy_partitions.rs	0.00% <0.00%> (ø)
core/consensus/src/impls.rs	42.48% <0.00%> (ø)
core/consensus/src/plane_helpers.rs	56.56% <75.00%> (ø)
core/simulator/src/client.rs	0.00% <0.00%> (ø)
core/shard/src/router.rs	0.00% <0.00%> (ø)
core/common/src/types/consensus/message.rs	58.97% <35.48%> (-1.42%)	⬇️
... and 1 more

... and 13 files with indirect coverage changes

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