Add format string validation

[ericsciple]

Validates format() function calls for:

• Invalid syntax (missing closing brace, empty placeholder, non-numeric placeholder)
• Argument count mismatch (placeholder references arg that doesn't exist)

I added this validation in the consumer for now (languageservice), instead of directly in expression parsing. This matches the current server behavior. Otherwise we need to reconcile xlang test differences. Eventually this validation should be added on the server as well during parse time, at which point we can move the validation here to expression parsing as well. The alternative approach was explored here.

[Copilot]

Pull request overview

This PR adds validation for format() function calls in GitHub Actions workflows to catch invalid format string syntax and argument count mismatches at the language service level.

Key changes:

• Implements format string parsing logic to validate placeholder syntax (numeric indices, escaped braces)
• Validates that placeholder indices don't exceed the number of provided arguments
• Integrates validation into the existing expression validation pipeline

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
languageservice/src/validate-format-string.ts	New module implementing format string validation logic ported from Go implementation, including AST traversal to find all format() calls
languageservice/src/validate.ts	Integrates format string validation into the expression validation flow and generates appropriate error diagnostics
languageservice/src/validate.format-string.test.ts	Comprehensive test suite covering unit tests for format string parsing and integration tests for workflow validation

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.