Feat: Comprehensive Input Validation

[akargi]

Closes #139

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Overview

Adds thorough input validation for all earn-quest contract functions to prevent invalid data and edge cases.

Features

• Validation helper functions for addresses, amounts, deadlines, strings, arrays, and status transitions
• Input sanitization and range checks
• Enforcement of string and array length limits
• Edge case handling (0, max values)
• Validation-specific tests

Acceptance Criteria

• All inputs validated before processing
• Invalid inputs return clear errors
• Edge cases handled
• String/array lengths enforced
• Time validations work correctly
• All validation rules tested

Impacted Files

• contracts/earn-quest/src/validation.rs
• contracts/earn-quest/src/quest.rs
• contracts/earn-quest/src/submission.rs
• contracts/earn-quest/src/lib.rs
• contracts/earn-quest/tests/test_validation.rs

Related Issue

• Issue Add Comprehensive Input Validation #139

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