Add WebhookSignature.generate_test_header_string helper

[vishwakt]

Summary

Adds a public WebhookSignature.generate_test_header_string() helper that mirrors stripe-node's generateTestHeaderString(), so users can sign test payloads without reaching for the underscore-prefixed _compute_signature internal.

Fixes #1697.

Implementation notes

• New method lives on WebhookSignature (next to EXPECTED_SCHEME, _compute_signature, and verify_header), accessed as stripe.WebhookSignature.generate_test_header_string(...).
• Python Optional keyword arguments are used instead of the Node SDK's options object, matching Python conventions while preserving the full Node parameter surface (payload, secret, timestamp, scheme, signature).
• Defaults: timestamp → int(time.time()), scheme → EXPECTED_SCHEME, signature → computed from payload + secret. Passing signature explicitly inserts it verbatim, mirroring Node.
• The internal generate_header helper in tests/test_webhook.py was refactored to delegate to the new public method. This proves the new API covers every existing test scenario and removes the duplicated signing logic from tests.
• New TestGenerateTestHeaderString class covers: defaults, round-trip through verify_header, custom timestamp/scheme, signature pass-through, and bad-secret failure.

Example usage

import stripe

header = stripe.WebhookSignature.generate_test_header_string(
    payload=payload_json,
    secret="whsec_test_secret",
)
event = stripe.Webhook.construct_event(payload_json, header, "whsec_test_secret")

Test plan

• just test — 4245 passed, 6 skipped
• just lint — clean
• just format-check — clean
• just typecheck — only pre-existing error in stripe/_http_client.py:681 (present on master, unrelated to this change)

[cla-assistant]

All committers have signed the CLA.

[Viicos]

The docstring uses a mix of markdown and RST syntax. I would suggest sticking to markdown to be consistent with the rest of the code base. You can specify parameters as:

Args:
    payload: ...
    secret: ...