Allow LedgerEntryType values in AccountObjects request for canonical ledger entry names

[Copilot]

High Level Overview of Change

Updates AccountObjects.type field to accept both AccountObjectType enum and LedgerEntryType enum values, supporting canonical ledger entry names added in rippled 2.4.0.

Context of Change

rippled 2.4.0 added support for canonical ledger entry names (XRPLF/rippled#5271), but the AccountObjects model only accepted AccountObjectType enum values, rejecting valid requests using LedgerEntryType.

# Previously only this worked:
AccountObjects(account=wallet.address, type=AccountObjectType.ESCROW)

# Now both work:
AccountObjects(account=wallet.address, type=LedgerEntryType.ESCROW)  # canonical ledger entry type
AccountObjects(account=wallet.address, type=AccountObjectType.ESCROW)  # original enum

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Tests (You added tests for code that already exists, or your new feature included in this PR)

Did you update CHANGELOG.md?

• No, this change does not impact library users

Test Plan

• Added unit tests for AccountObjectType and LedgerEntryType enum values in tests/unit/models/requests/test_account_objects.py
• Verified all 734 existing unit tests pass

Original prompt

---

This section details on the original issue you should resolve

<issue_title>account_objects model unnecessarily fails validation when using canonical ledger entry name in "type" field</issue_title>
<issue_description>rippled version 2.4.0 added support for canonical ledger entry names ( XRPLF/rippled#5271 ) but the AccountObjects request model hasn't been updated, so it rejects such usage.

example request:

response = client.request(AccountObjects(
            account=wallet.address,
            ledger_hash=ledger_hash,
            type="Escrow",
            marker=marker
        ))

Should be valid but fails with this error:

{'type': "type is <class 'str'>, expected typing.Optional[xrpl.models.requests.account_objects.AccountObjectType]"}
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes account_objects model unnecessarily fails validation when using canonical ledger entry name in "type" field #888

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[mvadari]

Update AccountObjects.type field to accept both AccountObjectType enum and str using Union[AccountObjectType, str]

@copilot it should actually be Union[AccountObjectType, LedgerEntryType]

[Copilot]

Update AccountObjects.type field to accept both AccountObjectType enum and str using Union[AccountObjectType, str]

@copilot it should actually be Union[AccountObjectType, LedgerEntryType]

Updated to use Union[AccountObjectType, LedgerEntryType] instead of str in commit 339a0e9. The type field now accepts both the original AccountObjectType enum and the LedgerEntryType enum for canonical ledger entry names.

[kuan121]

@mvadari This PR is in Draft state. Let us know when it's ready for review.

[mvadari]

Closing in favor of #895