feat: CommandBatch type and SendBatchAsync() for batched command execution

[Copilot]

Adds sequential batch command execution to IMediatorSendOnly — execute a list of void commands in order, stopping and propagating the first exception with no rollback.

New Types

• CommandBatch (Extensibility/CommandBatch.cs) — fluent builder that stores commands as type-erased Func<IMediatorSendOnly, CancellationToken, Task> delegates
• MediatorSendOnlyExtensions (Extensibility/MediatorSendOnlyExtensions.cs) — SendBatchAsync extension on IMediatorSendOnly

Usage

var batch = new CommandBatch()
    .Add(new CreateOrderCommand(items))
    .Add(new ReserveInventoryCommand(items))
    .Add(new SendConfirmationCommand(customerId));

await mediator.SendBatchAsync(batch, cancellationToken);
// On first failure: exception propagates, remaining commands skipped, no rollback

Key Design Decisions

• Add<TCommand> constrained to ICommand (void) — batch returns Task, responses are meaningless
• Commands stored as closed-over delegates at Add time, not re-resolved at execution time
• Commands list is internal to keep the execution contract encapsulated in SendBatchAsync

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.28%. Comparing base (ccdff8a) to head (5f63702).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #421      +/-   ##
==========================================
+ Coverage   91.85%   92.28%   +0.42%     
==========================================
  Files         146      148       +2     
  Lines        5547     5558      +11     
  Branches      522      523       +1     
==========================================
+ Hits         5095     5129      +34     
+ Misses        304      280      -24     
- Partials      148      149       +1     

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