fix: support escaped fields in point selections

[vs-li]

PR Description

Summary

Use flatAccessWithDatum when extracting point-selection values.

• Escaped fields (ex. "field": "y\\[foo\\]") now access datum["y[foo]"].
  • This change applies to point-selection and bound input selections.
• Existing point-selection behavior remains unchanged for fields without escaped chars.
• Added regression testing for point tuples and nearest bound selections.

Motivation

Point selections manually generated datum access with stringValue, which quoted escaped field paths without interpreting them:

datum["y\\[foo\\]"]

Other Vega-Lite compiler paths use shared field-access helpers that correctly generate:

datum["y[foo]"]

This caused point-selection behavior to differ from rendering, transforms, predicates, and other generated expressions.
As a result, point selections did not work as expected for marks using escaped field names.

History!

In 2019, #5351 changed selection field access to manual bracket lookup to support flattened fields such as nested.a. This fixed nested selections but bypassed the shared helper that interprets escaped field names.
Escaped bracket support was later added in #5730, but point selections did not inherit the fix when selection types were consolidated in #6927.

Checklist

• This PR is atomic (i.e., it fixes one issue at a time).
• The title is a concise semantic commit message (e.g. "fix: correctly handle undefined properties").
• npm test runs successfully
• For new features:
  • Has unit tests.
  • Has documentation under site/docs/ + examples.

Tips:

• https://medium.com/@greenberg/writing-pull-requests-your-coworkers-might-enjoy-reading-9d0307e93da3 is a nice article about writing a nice PR.
• Use draft PR for work in progress PRs / when you want early feedback (https://github.blog/2019-02-14-introducing-draft-pull-requests/).

[vs-li]

@domoritz This PR is ready for review as well! Was doing some self-highlight using selection and noticed a bug with escaped fields, so created this.

[domoritz]

Good catch. Thank you!