docs(projects): document unavailable project contract

[grevgeny]

Summary

• Document that missing and cross-company project IDs share the same 404 Not Found contract.
• Document sparse PATCH preservation and demonstrate that PATCH does not create an unavailable project.

Why

The public integration examples must match the hardened Ditio Core project GET/PATCH contract so partner implementations handle unavailable IDs consistently and do not treat PATCH as an upsert.

Changes

• Update the projects README with the unavailable-project and omitted-field behavior.
• Add GET and PATCH calls against a deterministic unavailable ID to ProjectsExample.cs.

Test plan

• dotnet build Ditio.Samples.csproj --no-restore — passed with 0 warnings and 0 errors.
• git diff --check — passed.

Risk

• Breaking change: No.
• Risk level: Low; documentation and sample-code changes only.
• Rollback: Revert commit 6f8162d.

Refs ditioas/timestyr#4125