Improve dynamic resolver power user mode error

[officialasishkumar]

Improves the error raised when dynamic resolver decorators are used without enabling power user mode, including the case where the config key is missing entirely.

Changes

• Replaced direct indexing of hamilton.enable_power_user_mode with a defaulted config lookup.
• Updated the error text to point users at .with_config({'hamilton.enable_power_user_mode': True}).
• Added regression coverage for both explicit False and missing power-user-mode config.

How I tested this

• uv run --with pytest pytest tests/function_modifiers/test_delayed.py::test_delayed_without_power_mode_fails tests/function_modifiers/test_delayed.py::test_delayed_without_power_mode_config_fails_with_helpful_error
• uv run ruff format --check hamilton/function_modifiers/delayed.py tests/function_modifiers/test_delayed.py
• uv run ruff check hamilton/function_modifiers/delayed.py tests/function_modifiers/test_delayed.py

Notes

Fixes #1135.

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.