Fix/panic safety (1/8)

[bnomei]

Linked Issue

Closes #

Description

Remove panic-prone code paths and add safe accessors to prevent runtime crashes in production use. This adds safe getters for JsonValue object keys and array indices, replaces scanner and expansion unwraps with safe handling or expect-with-context, and improves indexing error context. This is PR 1 of 8 in the intended sequence (1-8).

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test coverage improvement

Changes Made

• Added safe getters for JsonValue object keys and array indices; improved indexing error context.
• Replaced unwraps in scanner and path expansion with safe handling or expect-with-context.
• Added panic-safety tests for missing keys, wrong types, and out-of-bounds indices.

SPEC Compliance

• This PR implements/fixes spec compliance
• Spec section(s) affected:
• Spec version:

Testing

• All existing tests pass
• Added new tests for changes
• Tests cover edge cases and spec compliance

Tests run: cargo test

Pre-submission Checklist

• My code follows the project's coding standards
• I have run code formatting/linting tools
• I have added tests that prove my fix/feature works
• New and existing tests pass locally
• I have updated documentation if needed
• I have reviewed the TOON specification for relevant sections

Breaking Changes

• No breaking changes
• Breaking changes (describe migration path below)

Migration path:

Additional Context

Branch: fix/panic-safety
Intended PR order: 1-8 (this is PR 1 of 8)