[BUG] GreedyEncoder silently truncates sequences exceeding max_len without warning (#670)

#### Reference Issues/PRs
Fixes #669 

#### What does this implement/fix? Explain your changes.

`GreedyEncoder` in `pyaptamer/trafos/encode/_greedy.py` was silently
truncating sequences that exceeded `max_len` with no indication to the
user. This is inconsistent with how `clean_protein_seq` in
`_pseaac_utils.py` handles sequence modification — it issues a
`UserWarning` whenever it changes a sequence. From a chemistry
standpoint this is also risky since aptamer sequences fold into specific
3D structures like stem-loops and G-quadruplexes that are length
dependent, so a silent truncation could cause a sequence to encode a
structurally different conformation without the researcher knowing.

The fix adds a `UserWarning` when truncation occurs, following the exact
same pattern already established by `clean_protein_seq`.

#### What should a reviewer concentrate their feedback on?

- The placement of `warnings.warn()` inside `_transform()` and whether
`stacklevel=2` is correct for this call depth
- Whether the warning message is clear enough for end users

#### Did you add any tests for the change?

No new tests were added. The fix is a single warning call that follows
the existing pattern in the codebase.

#### Any other comments?

This was discussed with @satvshr on Discord who confirmed the fix is
welcome.

#### PR checklist
- [x] The PR title starts with either [ENH], [MNT], [DOC], or [BUG].
[BUG] - bugfix, [MNT] - CI, test framework, [ENH] - adding or improving
code, [DOC] - writing or improving documentation or docstrings.
- [ ] Added/modified tests
- [x] Used pre-commit hooks when committing to ensure that code is
compliant with hooks. Install hooks with `pre-commit install`.
To run hooks independent of commit, execute `pre-commit run --all-files`

Author: SanchitKS12

pyaptamer/trafos/encode/_greedy.py

@@ -1,5 +1,7 @@
 """Base transformation class."""
 
+import warnings
+
 import numpy as np
 import pandas as pd
 
@@ -116,6 +118,11 @@ def _transform(self, X):
                 # stop if we've reached max_len tokens
                 if max_len is not None and len(tokens) >= max_len:
                     tokens = tokens[:max_len]
+                    warnings.warn(
+                        "One or more sequence exceeds maximum length and was truncted ",
+                        UserWarning,
+                        stacklevel=2,
+                    )
                     break
 
             encoded_seqs.append(tokens)