Support metadata in credit notes

[nepalez]

Roadmap Task

• SPEC: Metadata on credit notes
• BE: Metadata on credit notes
• Canny request

Context

This PR adds Python client support for credit note metadata. This complements the backend implementation that enables customers like Mistral AI to flag credit notes synced to external systems.

Description

New methods on CreditNoteClient

Method	HTTP	Endpoint	Description
replace_metadata(id, metadata)	POST	/credit_notes/:id/metadata	Replaces all metadata
merge_metadata(id, metadata)	PATCH	/credit_notes/:id/metadata	Merges new keys into existing
delete_all_metadata(id)	DELETE	/credit_notes/:id/metadata	Removes all metadata
delete_metadata_key(id, key)	DELETE	/credit_notes/:id/metadata/:key	Removes a single key

Metadata format

Metadata is represented as a dict with string keys and optional string values:

client.credit_notes.replace_metadata(credit_note_id, {
    "foo": "bar",
    "baz": None
})

Changes

• lago_python_client/credit_notes/clients.py — added 4 new metadata methods
• lago_python_client/models/credit_note.py — added metadata field to CreditNote, CreditNoteUpdate, and CreditNoteResponse models
• lago_python_client/services/request.py — added send_patch_request for PATCH support
• tests/fixtures/credit_note.json — added metadata field to fixture
• tests/test_credit_note_client.py — added tests for all 4 metadata methods with null value handling