feat: add to_map expression constructor

[dwallener]

First time looking/working with Daft code...any process errors are strictly mine.

Changes Made

Fixes #6885

Summary

• Add a daft.functions.to_map constructor for key/value expression pairs.
• Support keyword arguments as string keys.
• Add expression tests for positional pairs, keyword pairs, mixed use, map_get, coercion, and invalid inputs.

Tests

• make build
• DAFT_RUNNER=native pytest -q tests/expressions/test_to_map.py
• DAFT_RUNNER=native pytest -q tests/expressions/test_to_list.py tests/recordbatch/struct/test_to_struct.py tests/expressions/test_to_map.py
• DAFT_RUNNER=native pytest -q tests/recordbatch/test_from_py.py tests/expressions/test_to_map.py

AI usage

Used AI assistance to draft the implementation and tests; verified locally with the commands above.

Related Issues

[greptile-apps]

Greptile Summary

This PR adds a to_map expression constructor. The main changes are:

• Public Python export through daft.functions.
• Python helper for positional and keyword map entries.
• Rust scalar UDF registration and map physical construction.
• Tests for construction, lookup, coercion, and invalid inputs.

Confidence Score: 4/5

The new map constructor needs a fix for empty input batches with literal entries.

• Literal keys or values can stay length 1 when the input row count is 0.
• That can make a zero-row select return a one-row map result.
• The public API and registration paths otherwise follow the existing function patterns.

src/daft-functions/src/to_map.rs

Important Files Changed

Filename	Overview
daft/functions/init.py	Exports to_map from the public functions module.
daft/functions/struct.py	Adds the Python wrapper that validates pair counts and forwards entries to the builtin scalar function.
src/daft-functions/src/lib.rs	Registers the new map scalar function in the miscellaneous function module.
src/daft-functions/src/to_map.rs	Builds map arrays from key/value inputs, with a zero-row literal cardinality issue in the broadcast path.
tests/expressions/test_to_map.py	Adds coverage for common map construction paths but not empty input batches.

_{Reviews (1): Last reviewed commit: "Add to_map expression constructor" | Re-trigger Greptile}

[dwallener]

All PR-related checks now appear addressed. The remaining failures seem to come from IO integration flakiness unrelated to this change:

• integration-test-io (3.10, native)
• integration-test-io (3.10, ray)
• aggregate integration-tests required check

The IO logs show Hugging Face/S3 external access failures, e.g. https://huggingface.co/api/datasets/stanfordnlp/imdb/parquet returning HTTP 400 and transient S3 GET timeouts for daft-oss-public-data.

This PR only adds to_map expression construction and does not touch IO/Hugging Face/S3 code. I reran the failed checks and they still failed in the same unrelated area.