refactor: Issue deprecation warnings for IO-functions

[Oliver Borchert (borchero)]

Motivation

Contributes to #367.

[Copilot]

Pull request overview

This PR implements the deprecation-warning layer needed for the planned I/O API reduction (Issue #367), by introducing a centralized warning helper/decorator and wiring it into Schema/Collection I/O entry points, plus adding documentation and tests to codify the new behavior.

Changes:

• Add issue_deprecation_warning() and a deprecated decorator abstraction to consistently emit DeprecationWarnings that point at user call sites.
• Mark Schema I/O methods and Collection Delta I/O methods as deprecated, and emit a deprecation warning when reading parquet collections with validation != "skip".
• Add dedicated deprecation tests and adjust pytest warning filters so existing tests aren’t broken by the new warnings.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
dataframely/_deprecation.py	Introduces issue_deprecation_warning and a cross-version deprecated decorator implementation.
dataframely/schema.py	Applies deprecation decorators (and docstring notes) to Schema parquet/delta I/O methods.
dataframely/collection/collection.py	Emits deprecation warnings for implicit validation on parquet reads/scans; deprecates delta I/O methods.
tests/test_deprecation.py	Adds/updates tests to assert the new deprecation warnings and stacklevel behavior.
pyproject.toml	Adds warning filters to ignore the new I/O deprecations outside dedicated warning-assertion tests.
docs/guides/migration/index.md	Updates migration guidance from FutureWarnings to DeprecationWarnings.
docs/guides/features/serialization.md	Adds a prominent warning block documenting the I/O deprecations and migration guidance.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (fd0386b) to head (5a4f19d).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #368   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files           56        56           
  Lines         3489      3506   +17     
=========================================
+ Hits          3489      3506   +17     

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