Add get_by_name to Users, Groups, and Schedules endpoints (unique-by-name types)

[jacalata]

Sites already has get_by_name(site_name) -> SiteItem. Three other endpoints have server-enforced unique names and would benefit from the same shape:

• Users — usernames unique per site.
• Groups — group names unique per site.
• Schedules — server throws ScheduleNameExistsException on duplicate name.

Proposal:

# UsersEndpoint
def get_by_name(self, username: str) -> UserItem | None:
    matches = list(self.filter(name=username))
    return matches[0] if matches else None

This formalizes a naming pattern across the API surface:

• get_by_* returns a single item (or None/raises when not found). Use when the lookup key is unique on the server.
• filter(...) returns a lazy QuerySet[T] for chaining or when multiplicity is possible.

Callers with unique-name types get a natural T | None shape. Callers with multi-name endpoints (workbooks/datasources/views/flows/projects) continue to use .filter(name=name).

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