Add declarative Marten data requirements for handler middleware

[jeremydmiller]

Summary

• Implement IMartenDataRequirement support in Before/Validate middleware methods that throw RequiredDataMissingException on failure
• Fix bug where MustNotExist was creating DocumentExists instead of DocumentDoesNotExist
• Expand RequirementResultContinuationPolicy to handle all Before method names (not just Validate/ValidateAsync)
• Add MartenDataRequirementFrame implementing IBatchableFrame for efficient batch querying when combined with entity loads
• Add MartenDataRequirementContinuationStrategy detecting IMartenDataRequirement and IEnumerable<IMartenDataRequirement> returns
• Support three handler patterns: single requirement, multiple requirements via IEnumerable, and requirement + [Entity] attribute

Test plan

• Single IMartenDataRequirement (MustExist) happy + sad path
• IEnumerable<IMartenDataRequirement> (MustExist + MustNotExist) happy + 2 sad paths
• Single requirement + [Entity] attribute happy + sad path (validates batching)
• All 7 tests passing
• MartenTests regression check (375/381 pass, failures are pre-existing/flaky)
• CoreTests middleware and validation tests pass

🤖 Generated with Claude Code

[jeremydmiller]

I hate the usability on this one. Not thinking this goes in anytime soon.

[jeremydmiller]

Notes

As an easier to use alternative, let's keep the current syntax, but add attribute alternatives for declaring that a document must or must not exist first:

[DocumentDoesNotExist<User>(nameof(AddUser.UserId))]
public void Handle(AddUser command)
{

}

// assume a property named "Id" or "UserId" on the command
[DocumentExists<User>]
public void Handle(PromoteUser command)
{

}