fix: incorrect index implementation in FileCalendarStorage

[Srujanrana07]

Description

Fixes incorrect behavior in FileCalendarStorage.index() where a value
present in the calendar could still raise an IndexError.

The previous implementation used NumPy comparison on a Python list,
which resulted in incorrect boolean evaluation and failure to locate
existing values.

This change replaces it with Python’s built-in list.index() method,
ensuring correct lookup behavior.

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Motivation and Context

Fixes #2192

The current implementation:

np.argwhere(calendar == value)

fails because calendar is a Python list, and list == value returns
a single boolean instead of an element-wise comparison.

This leads to:

• Incorrect IndexError even when value exists
• Wrong exception type (should be ValueError if not found)

This fix aligns behavior with Python's standard list.index().

How Has This Been Tested?

• Pass the test by running: pytest qlib/tests/test_all_pipeline.py under upper directory of qlib.
• Verified logic manually for correct index lookup behavior

Types of changes

• Fix bugs
• Add new feature
• Update documentation

[SunsetWolf]

Hi, @Srujanrana07 , Thanks for the contribution! Really appreciate your work on this.