Fix settings access for groups with a non-admin bot

[iamwavecut]

Opening group settings fails with HTTP 500 when a non-admin bot receives CHAT_ADMIN_REQUIRED from Telegram getChatMember, even for an administrator who has permission to manage settings.

On this specific Telegram 400 response, resolve the caller from a fresh getChatAdministrators response. Both the Settings WebApp and the group settings command use this adapter. Existing owner/admin permission checks remain in place. A missing caller, failed fallback, or unrelated Telegram error does not grant access.

Validation:

• Reproduced the regression before the fix with a local Bot API HTTP fixture.
• cargo test --locked -p openplotva-app: 1,411 passed, 7 ignored, 0 failed.
• cargo clippy --locked --workspace --all-targets -- -D warnings: passed.
• cargo fmt --all -- --check and git diff --check: passed.
• Regression coverage includes owners, administrators with and without promotion rights, absent callers, fallback errors, successful direct lookups, and unrelated Telegram errors.

[github-actions]

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