idl/spec: avoid panic when parsing invalid array types

[yukikm]

Fixes a panic/DoS in IdlType::from_str when parsing malformed array-like type strings (e.g. [u832]).

Changes: replace unwrap() in array parser with error-returning logic; add regression tests.

Write-up: https://paste.rs/yHF7r

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Pull request overview

This PR fixes a panic/DoS vulnerability in the IdlType::from_str parser when handling malformed array-like type strings. The issue was caused by unwrap() calls that would panic on invalid input such as "[u832]" (missing semicolon separator).

Changes:

• Refactored array_from_str helper function to return anyhow::Result<IdlType> instead of panicking
• Replaced all unwrap() calls with proper error handling using ok_or_else and map_err
• Added validation for empty array lengths and malformed nested array syntax
• Added two regression tests to prevent future panics on invalid input

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[jamie-osec]

Closing in favour of #4029