feat: Add string trim function

[Lucas61000]

Changes Made

Add trim() function to Daft's string functions.

Related Issues

#3792

[greptile-apps]

Greptile Summary

This PR adds a trim() string function to Daft, which strips leading and trailing whitespace from UTF-8 strings. It follows the exact same layered pattern as the existing lstrip and rstrip functions: a Rust ScalarUDF (Trim) delegates to Rust's built-in str::trim(), exposed through a Python daft.functions.trim() top-level function, a SeriesStringNamespace.trim() method, and an Expression.trim() convenience method.

• The implementation is correct and complete across all layers (Rust UDF, Python function, Series namespace, Expression method, __init__ export, and a basic test).
• Two style-level concerns are flagged: (1) the trim() method in expressions.py uses an inline from daft.functions import trim import, consistent with the existing file pattern but in violation of the project's import-placement convention; (2) the PR adds a third standalone strip function (trim) alongside lstrip and rstrip, which diverges from the project preference for a single parametrized function covering all variants.

Confidence Score: 4/5

• This PR is safe to merge; no logic bugs or security issues were found.
• The implementation faithfully mirrors the established lstrip/rstrip pattern at every layer, uses correct Rust stdlib semantics, handles None values, and is covered by a test. The score is 4 rather than 5 due to two style-level convention violations (inline import and preference for parametrized strip functions) that are worth addressing before or after merge.
• No files require special attention; style issues are in daft/expressions/expressions.py and daft/functions/str.py.

Important Files Changed

Filename	Overview
daft/expressions/expressions.py	Adds trim() method to the Expression class, following the same pattern as lstrip/rstrip. Uses an inline import which violates the project's import convention.
daft/functions/init.py	Exports trim from the str module and adds it to __all__. The __all__ entry is correctly placed in alphabetical order.
daft/functions/str.py	Adds a trim() function with docstring and doctest, consistent with lstrip/rstrip. The function is a third separate strip variant rather than using a parametrized approach.
daft/series.py	Adds trim() to SeriesStringNamespace, consistent with existing lstrip/rstrip methods.
src/daft-functions-utf8/src/trim.rs	New Rust Trim scalar UDF, closely mirroring lstrip.rs/rstrip.rs. Uses Rust's standard str::trim() which strips both leading and trailing Unicode whitespace. No clippy suppressions; no logic issues found.
src/daft-functions-utf8/src/lib.rs	Registers the new trim module, re-exports it, and adds Trim to the function module. Changes are minimal and correct.
tests/recordbatch/utf8/test_trim.py	Single test covering None passthrough and various whitespace characters on both sides. Matches the coverage level of test_lstrip.py and test_rstrip.py.

Sequence Diagram

sequenceDiagram
    participant User as Python User
    participant Expr as Expression.trim()
    participant Fn as daft.functions.trim()
    participant Series as SeriesStringNamespace.trim()
    participant Rust as Trim ScalarUDF (Rust)

    User->>Expr: col("x").trim()
    Expr->>Fn: trim(self)
    Fn->>Fn: _call_builtin_scalar_fn("trim", expr)
    Fn-->>User: Expression (lazy plan node)
    User->>Series: series.str.trim()
    Series->>Series: _eval_expressions("trim")
    Series->>Rust: Trim::call(inputs)
    Rust->>Rust: val.trim()
    Rust-->>Series: DaftResult[Series]
    Series-->>User: trimmed Series

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_{Last reviewed commit: bc0b6ae}

[Lucas61000]

Hi @universalmind303 @srilman , could you please review this PR when you have time? I’d appreciate your feedback on how to best handle the trim and strip functions.