feat: Add string validation support (pattern, minLength, maxLength)

[domdomegg]

Summary

Implements detection of JSON Schema string validation constraints: pattern, minLength, and maxLength.

Changes

Pattern Support

• PatternAdd/Remove/Change change types
• Pattern changes conservatively treated as breaking (determining if one regex is a subset of another requires complex AST analysis or fuzzing, which is out of scope per issue discussion)

MinLength Support

• MinLengthAdd/Remove/Change change types
• Smart breaking logic:
  • Increasing minLength: breaking (rejects previously valid shorter strings)
  • Decreasing minLength: non-breaking (accepts more strings)

MaxLength Support

• MaxLengthAdd/Remove/Change change types
• Smart breaking logic:
  • Decreasing maxLength: breaking (rejects previously valid longer strings)
  • Increasing maxLength: non-breaking (accepts more strings)

Test Coverage

14 comprehensive test fixtures covering:

• Pattern: add, remove, change, unchanged
• MinLength: add, remove, increase (breaking), decrease (non-breaking), unchanged
• MaxLength: add, remove, decrease (breaking), increase (non-breaking), unchanged

Examples

# Pattern add (breaking)
{"path":"","change":{"PatternAdd":{"added":"^[a-z]+$"}},"is_breaking":true}

# MinLength increase 3→5 (breaking)
{"path":"","change":{"MinLengthChange":{"old_value":3,"new_value":5}},"is_breaking":true}

# MinLength decrease 5→3 (non-breaking)
{"path":"","change":{"MinLengthChange":{"old_value":5,"new_value":3}},"is_breaking":false}

# MaxLength decrease 10→5 (breaking)
{"path":"","change":{"MaxLengthChange":{"old_value":10,"new_value":5}},"is_breaking":true}

Fixes #23 Fixes #50