ORLEANS0020 should offer a code fix to create OrleansContracts.txt

[ReubenBond]

Description

When contract analysis is enabled without an OrleansContracts.txt file, ORLEANS0020 is reported but offers no code fix.

The code-fix provider excludes ORLEANS0020 from FixableDiagnosticIds, even though its implementation already contains logic for creating an additional document.

Reproduction

Enable contract analysis in a project containing a grain interface:

<PropertyGroup>
  <EnableOrleansContractsAnalyzer>true</EnableOrleansContractsAnalyzer>
</PropertyGroup>

Delete or omit OrleansContracts.txt, then build.

Actual behavior

ORLEANS0020 is reported, but the file must be manually created before the remaining contract code fixes become available.

Expected behavior

Offer a code fix which creates OrleansContracts.txt, preferably populating its deterministic generated header and current contract declarations.

Version

Microsoft Orleans 10.3.0

[ReubenBond]

Reopening with a dotnet format reproduction from the Sydney repository.

Updated reproduction

1. Enable the Orleans contract analyzer in a project which contains Orleans contracts.
2. Ensure that OrleansContracts.txt does not exist.
3. From the repository root, run:

dotnet format PATH_TO_PROJECT_OR_SOLUTION analyzers --severity info --diagnostics ORLEANS0016 ORLEANS0017 ORLEANS0018 ORLEANS0019 ORLEANS0020 ORLEANS0022 ORLEANS0023 ORLEANS0024

Actual behavior

The command exits successfully and reports a source file as formatted, but does not create OrleansContracts.txt. Regeneration begins working only after manually creating a format-2 seed manifest.

Expected behavior

Applying the ORLEANS0020 fix through dotnet format creates and populates the configured manifest without requiring a seed file.

Observed in C:\dev\Sydney after the fix from #10869 was merged.