Document validate.proto invariants in CONTRIBUTING

[EaganVP]

Add a new section to CONTRIBUTING.md explaining the structural invariants of validate.proto that contributors, maintainers, and implementers must uphold.
The contributing guide now documents what the checker verifies: field-to-rules type mapping, field name consistency, extension ranges, example fields, and CEL rule requirements.

[github-actions]

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Mar 31, 2026, 6:56 PM

[timostamm]

This is great, thank you.

Can you also add a section about the CEL environment? It's important that any CEL expression in validate.proto honors the environment provided by Protovalidate. On top of CEL's standard functions, we add:

• The strings extension, most comprehensively documented here.
• The variables now, this, rule, and rules.
• The double, int, string, and list functions, and bytes overloads for contains, endsWith, and startsWith.

The variables and functions are documented on protovalidate.com.

Rules in validate.proto cannot use other functions and variables. The tool ensures that by defining the function and variable signatures in tools/internal/protovalidate-check/cel.go. A contribution that adds a new signature must add it to this definition.