fix(commerce-elastic-path): load complete account roster

[mzaintariq]

Summary

Closes #609.

Unpaged account roster reads now return the complete organisation roster instead of only the first 100 accounts.

The server-side roster endpoint walks Elastic Path account-token pages sequentially at the platform page size of 100 and returns the existing:

{
  accounts,
  total,
}

shape once all pages have been collected.

Explicit paged requests using { limit, offset } remain single-page operations.

If a later page fails, the whole roster request uses the existing account-token failure path rather than returning a partial roster. For a large roster, this means a later-page failure now results in the Provider's existing empty-roster fallback instead of the first page alone. This is intentional: a partial list beside a larger total shouldn't be treated as a complete roster.

No changes were made to the Account Provider, identity client, login flow, selection/retry behavior, or Studio surface.

Testing

• Unpaged roster with <=100 accounts uses one Elastic Path request
• 150-account roster loads offsets 0 and 100 and returns all accounts
• Explicit paged roster requests remain single-page
• Later-page failure returns no partial roster
• Tokens are not exposed in the roster response
• Provider publishes a >100 account roster from a single browser-side roster request
• Covers a middle page whose raw rows are all rejected by validation and still continues to later valid pages
• Account tests: 130 passed
• Auth plugin tests: 273 passed
• Full Elastic Path Jest suite: 2546 passed
• tsc --noEmit: clean