LocalAppAuth stores authentication data for local apps. When a Bitrix24 event or placement request
arrives, the raw auth payload contains server_endpoint — the OAuth server URL that issued the token
(e.g. oauth.bitrix24.tech for east, oauth.bitrix.info for west).

Previously LocalAppAuth did not store this URL, so any consumer reconstructing Credentials
from a saved LocalAppAuth had to hard-code or guess the OAuth server via DefaultOAuthServerUrl::default(),
which breaks multi-region deployments.

Changes:

• Added oauthServerUrl: string constructor parameter to LocalAppAuth
• Added getOAuthServerUrl(): string getter
• toArray() serialises it as oauth_server_url
• initFromArray() reads oauth_server_url with a fallback to DefaultOAuthServerUrl::default() for backward compatibility with files written by older SDK versions
• Added unit tests covering constructor, getter, round-trip, backward-compat fallback, and data-provider variants (east / west / custom)

Test plan

• make lint-cs-fixer — passed
• make lint-rector — passed (auto-fix applied: variable rename by Rector)
• make lint-phpstan — passed
• make lint-deptrac — passed (0 violations)
• make test-unit — passed (661 tests, 1136 assertions)

Closes #385

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