fix: handle missing struct field in const evaluation instead of panicking

[orizi]

Summary

Fixed a panic in constant evaluation when struct literals have missing fields by returning a ConstValue::Missing instead of calling expect(). Added a test case to verify proper error handling for statement-level const struct literals with missing fields.

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Type of change

Please check one:

• Bug fix (fixes incorrect behavior)
• New feature
• Performance improvement
• Documentation change with concrete technical impact
• Style, wording, formatting, or typo-only change

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Why is this change needed?

The constant evaluator was panicking when encountering struct literals with missing fields during constant evaluation. While semantic validation should catch these cases, the evaluator should handle them gracefully rather than crashing with an expect() call.

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What was the behavior or documentation before?

The constant evaluator would panic with "Should have been caught by semantic validation" when processing struct literals with missing fields during constant evaluation.

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What is the behavior or documentation after?

The constant evaluator now gracefully handles missing struct fields by returning ConstValue::Missing and continues processing, allowing semantic validation to report the appropriate error messages to the user.

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Related issue or discussion (if any)

Fixes #9790

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Additional context

The fix maintains the existing error reporting behavior while preventing crashes in the constant evaluator. The test case demonstrates that proper error messages are still generated for missing struct fields in const declarations.

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Note

Low Risk
Low risk: small, localized change in constant evaluation that replaces a panic with a ConstValue::Missing fallback, plus a targeted diagnostic test.

Overview
Fixes a crash in const evaluation for struct literals with omitted fields by replacing an expect() with a safe fallback to ConstValue::Missing(skip_diagnostic()), allowing compilation to continue and diagnostics to surface normally.

Adds a regression test covering statement-level const declarations with a struct literal missing a field, asserting the missing-member error (and unused-const warning) is reported instead of panicking.

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[orizi]

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• fix: handle mismatched generic param kinds in substitution rewriter instead of panicking #9815
• fix: allow statement-level consts in named argument shorthand instead of panicking #9814
• fix: use Terminal::text() for modifier text to avoid panic on malformed attribute args #9813
• fix: handle missing struct field in const evaluation instead of panicking #9812 👈 (View in Graphite)
• fix: emit diagnostic for unsupported extern fn in const evaluation instead of panicking #9811
• fix: emit diagnostic for non-type numeric literal suffix instead of panicking #9810
• fix: return None for macro rule with missing param kind instead of panicking #9809
• fix: handle missing return type context in loop body instead of panicking #9808
• fix: emit diagnostic for member access on unreduced impl type instead of panicking #9807
• fix: emit diagnostic for unsupported items in statement position instead of panicking #9806
• main

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[reviewable-StarkWare]

This change is 

[eytan-starkware]

@eytan-starkware reviewed 2 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on TomerStarkware).

[eytan-starkware]

@eytan-starkware made 1 comment.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on orizi and TomerStarkware).

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crates/cairo-lang-semantic/src/items/constant.rs line 728 at r1 (raw file):

                                .find(|(_, member_id)| m.id == *member_id)
                                .map(|(expr_id, _)| self.evaluate(*expr_id))
                                // A missing field should have been caught by semantic validation.

Comment is a misleading. It was almost certainly caught, but allowed to continue processing