Fix/strict validation (3/8)

[bnomei]

Linked Issue

Closes #

Description

Complete strict-mode validation required by TOON v3.0 and adjust related non-strict behavior. This enforces array length and delimiter consistency checks, validates unquoted key rules in strict mode, updates indentation/tab handling by strictness, and fixes the CLI trailing newline plus max depth error messaging. This is PR 3 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

• Enforce strict checks for array length, delimiter consistency, and unquoted key rules.
• Adjust strict vs non-strict behavior for indentation tabs and length mismatches; fix CLI trailing newline.
• Fix max depth validation messaging and wire delimiter consistency validation.

SPEC Compliance

• This PR implements/fixes spec compliance
• Spec section(s) affected: array lengths, delimiters, unquoted keys, output formatting
• Spec version: v3.0

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/strict-validation
Intended PR order: 1-8 (this is PR 3 of 8)