feat: Support docstring-styled column documentation

[Copilot]

Enables documenting schema columns using docstrings placed immediately after column definitions, allowing structured documentation that can be programmatically accessed and preserved through serialization.

Changes

Docstring extraction

• Added AST parser in _base_schema.py to extract docstrings following column assignments
• Integrated extraction into SchemaMeta.__new__() to capture docstrings at class creation

Column class updates

• Added doc attribute to Column.__init__()
• Modified serialization to preserve doc in as_dict()/from_dict()
• Updated matches() to compare doc values
• Extended __repr__() to display non-null doc values

Testing

• Added 11 tests covering extraction, serialization, inheritance, multiline strings, and edge cases

Usage

class HouseSchema(dy.Schema):
    """Schema for real estate data"""
    
    zip_code = dy.String(nullable=False, min_length=3)
    """Postal code where the property is located"""
    
    num_bedrooms = dy.UInt8(nullable=False)
    """Number of bedrooms, excluding living areas"""

# Access programmatically
HouseSchema.columns()["zip_code"].doc  # "Postal code where..."

# Preserved in serialization
serialized = HouseSchema.serialize()
deserialized = deserialize_schema(serialized)
assert deserialized.columns()["zip_code"].doc == "Postal code where..."

# Visible in repr
print(HouseSchema)  
# [Schema "HouseSchema"]
#   Columns:
#     - "zip_code": String(min_length=3, doc='Postal code where...')

Limitations

Docstrings are extracted via inspect.getsource(), so dynamically created schemas (via exec()) won't have docstrings available. This matches standard Python docstring behavior.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Support for docstring styled column documentation</issue_title>
<issue_description>Feature request to allow for columns level docstrings in the following form:

class HouseSchema(dy.Schema):
    """Explanation of what my House Schema is really representing"""
    
    zip_code = dy.String(nullable=False, min_length=3)
    """A zip_code is a ..."""
    
    num_bedrooms = dy.UInt8(nullable=False)
    """Counts the rooms in a house not including the living room, kitchen and ..."""

    etc

This could potentially allow for printing of schema documentation notes into a standardised/centralised format (e.g., to support with creating data documentation)</issue_description>

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

• Fixes Support for docstring styled column documentation #132

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[codecov]

Codecov Report

❌ Patch coverage is 97.82609% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.96%. Comparing base (78bb16d) to head (cda1f70).

Files with missing lines	Patch %	Lines
dataframely/_base_schema.py	96.96%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #261      +/-   ##
===========================================
- Coverage   100.00%   99.96%   -0.04%     
===========================================
  Files           53       53              
  Lines         3098     3141      +43     
===========================================
+ Hits          3098     3140      +42     
- Misses           0        1       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.