Use DEFAULT_DESTINATION_REF constant in order_request.py

[Copilot]

Related Issue(s):

Addresses feedback from #1201 (comment #1201 (comment))

Proposed Changes:

For inclusion in changelog (if applicable):

1. N/A - internal refactoring only

Not intended for changelog:

1. Import and use DEFAULT_DESTINATION_REF constant from planet.clients.destinations in planet/order_request.py instead of hardcoding 'pl:destinations/default'

Diff of User Interface

Old behavior:

# planet/order_request.py hardcoded the destination reference
def default_destination(path_prefix: Optional[str] = None) -> dict:
    parameters: Dict[str, Any] = {'ref': 'pl:destinations/default'}
    ...

New behavior:

# planet/order_request.py now uses the constant for consistency
from .clients.destinations import DEFAULT_DESTINATION_REF

def default_destination(path_prefix: Optional[str] = None) -> dict:
    parameters: Dict[str, Any] = {'ref': DEFAULT_DESTINATION_REF}
    ...

Both planet/order_request.py and planet/subscription_request.py now use the same constant, establishing single source of truth.

PR Checklist:

• This PR is as small and focused as possible
• If this PR includes proposed changes for inclusion in the changelog, the title of this PR summarizes those changes and is ready for inclusion in the Changelog.
• I have updated docstrings for function changes and docs in the 'docs' folder for user interface / behavior changes
• This PR does not break any examples or I have updated them

(Optional) @mentions for Notifications:
@asonnenschein

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