feat: use custom labels for block parent input labels

[maribethb]

The basics

• I validated my changes

The details

Resolves

Fixes #9837

Proposed Changes

Updates the getParentInputLabel to use the following logic:

If an input has a custom label, the custom label will be prepended
to the first child block connected to that input

If an input does not have a custom label, the input's fallback
label determined from the field row will be prepended to the
child block's label only if the following are true:

• the parent block has at least one statement input
• the child block in question is not attached to the first
  statement input of the parent block (in this case, the label
  would be redundant with the parent block's label)

Add Begin prefix to the label if the parent input is a statement input.

Reason for Changes

Improving screenreader experience for custom blocks

Test Coverage

Added tests (partially written by AI)

Documentation

Additional Information