Update mode.py to warn in `Mode.FUNCTIONS` access vs. in `__new__`

[boydgreenfield]

Currently, any import of instructor raises a DeprecationWarning. This is noisy and also makes it impossible to have test suites raise an error on DeprecationWarning. Example code:

import warnings
warnings.simplefilter("default")

import instructor  # raises `DeprecationWarning: FUNCTIONS is deprecated and will be removed in future versionss enum_member = enum_class._new_member_(enum_class, *args)`

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:rocket: This description was created by Ellipsis for commit 90edc70cefc2ee9f30870f5f510ad2f33ace54eb

Summary:

This PR moves the deprecation warning for FUNCTIONS in Mode class to only trigger when accessed, reducing import-time noise.

Key points:

• Moved DeprecationWarning for FUNCTIONS from __new__ to __getattribute__ in a Mode metaclass.
• Reduces warning noise at import time.
• Allows better handling of DeprecationWarning in test suites.

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Generated with :heart: by ellipsis.dev

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:rocket: This description was created by Ellipsis for commit 9ef6f1aa5d17baeada3febcc5fb7ef98813d86ac

Summary:

This PR moves the deprecation warning for FUNCTIONS in the Mode class to only trigger when accessed, reducing import-time noise.

Key points:

• Moved DeprecationWarning for FUNCTIONS from __new__ to __getattribute__ in Mode class using _WarnOnFunctionsAccessEnumMeta metaclass.
• Reduces warning noise at import time.
• Allows better handling of DeprecationWarning in test suites.

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Generated with :heart: by ellipsis.dev