[1] feat: add response validation support

[romanindev]

Introduce response validation to ensure safe and predictable integrations.

Support:

• schema-based validation (initially generic interface)
• custom validation function
• validation executed after response parsing

Config example:

client.get('/users', {
  responseSchema: schema
})

or

client.get('/users', {
  validate: (data) => { ... }
})

Acceptance criteria

• validation runs after parsing
• validation can throw structured error
• validation can be optional per request