Add batch() method to CustomerIO track client

[hownowstephen]

Summary

• Adds batch() method to CustomerIO class wrapping POST /api/v2/batch
• Accepts a list of operation dicts, each with type, action, identifiers, and action-specific fields
• Validates that operations list is non-empty

Example

cio.batch([
    {
        "type": "person",
        "action": "identify",
        "identifiers": {"id": "1"},
        "attributes": {"name": "Alice"},
    },
    {
        "type": "person",
        "action": "event",
        "identifiers": {"id": "1"},
        "name": "purchase",
    },
])

Closes #96

Test plan

• test_batch_call — verifies correct URL (/api/v2/batch), method (POST), auth, content-type, and body structure
• Empty/None operations raises CustomerIOException
• Full test suite passes (29 tests)
• Lint passes

---

Note

Low Risk
Low risk: adds a new client method and corresponding unit test without altering existing request flows; main risk is incorrect endpoint/URL construction for non-default ports.

Overview
Adds a new CustomerIO.batch(operations) helper that posts a list of Track API v2 operations to POST /api/v2/batch, with a guard that rejects empty/None operation lists.

Introduces test_batch_call to assert the request method, auth header, content type, URL suffix, and payload shape, and to verify invalid inputs raise CustomerIOException.

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