Add client meta add endpoint

[Arittra-Bag]

Closes: #13

[Arittra-Bag]

Verified against live WP Cloud client meta API.

Input tested:

• add_meta("webhook_url", "https://example.com/sdk-add-meta-test-1") when webhook_url was absent.
• add_meta("webhook_url", "https://example.com/sdk-add-meta-test-2") again while the key already existed.
• set_meta("webhook_url", original_value) to restore the original webhook URL.

Observed:

• First add_meta() created the key successfully.
• Duplicate add_meta() also succeeded and overwrote the value.
• Original webhook_url was restored successfully.

Conclusion:
add_meta() correctly hits the new /client-meta/{client}/{key}/add endpoint, but live API behavior for duplicate webhook_url is overwrite/upsert, not InvalidRequestError. I’m leaving SDK behavior API-authoritative rather than adding client-side duplicate checks.

[Copilot]

Pull request overview

Adds the client metadata add operation so SDK consumers can create metadata keys without overwriting existing values.

Changes:

• Adds ClientClient.add_meta(...) for POST /client-meta/{client}/{key}/add.
• Clarifies set_meta(...) documentation as update/overwrite behavior and cross-references add_meta.

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[Arittra-Bag]

[1] add_meta uses /add and succeeds
ok
[2] add_meta surfaces API 400 as InvalidRequestError
ok: [400] Meta key already exists
[3] set_meta still uses /update
ok
[4] docstrings cross-reference
ok

I also ran a live WP Cloud smoke test. add_meta() created webhook_url when absent, set_meta() updated it, and the original webhook value was restored afterward.

Please note: live WP Cloud currently treats duplicate add for webhook_url as overwrite/upsert rather than returning 400. Since the SDK correctly calls the documented /add endpoint and inherited error handling already surfaces API 400s as InvalidRequestError, I kept the SDK API-authoritative and did not add client-side duplicate checks, as also mentioned in the previous comment - #22 (comment).