respecting ordered sequence while partial update is fixed

[Natgho]

refs #6202

[Natgho]

I considered including the ticket ID in the Description section of the test section, but decided it would be excessive. If you want it included, just let me know.

[ProstoSerghei]

Review: ListField.get_value partial + indexed keys handling

Summary

This PR modifies ListField.get_value() to correctly handle indexed HTML form keys
(e.g. field[0], field[1]) during partial=True updates and adds a regression test
covering that scenario.

The problem being addressed is valid, and the added test demonstrates a real edge case.
However, the change alters the evaluation order inside get_value() in a way that may
impact backward compatibility and deserves closer scrutiny.

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1. Behavioral Changes

Previously, the logic flow was effectively:

1. If HTML input → attempt parse_html_list
2. Otherwise → fall back to dictionary.get(self.field_name, empty)
3. Respect partial=True semantics when the field is absent

In the new implementation:

1. Parse indexed HTML list
2. If found → return
3. If partial=True → return empty
4. Then check plain key in dictionary
5. Finally return fallback

This reorders evaluation and introduces earlier short-circuit returns.

Concern

For partial=True, the method may now return empty earlier than before,
preventing fallback to dictionary.get(self.field_name, empty) in some cases.

Please clarify whether this preserves behavior for:

• Non-indexed list submission in multipart/form-data
• Standard HTML form submission without indexed keys
• Mixed cases (both field and field[0] present)

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2. Partial Update Semantics

The new logic explicitly prioritizes indexed HTML keys before evaluating
plain field names and partial fallback.

Questions:

• Was the previous fallback behavior during partial updates considered incorrect?
• Have you verified no regression occurs when a non-indexed list is submitted
  with partial=True?

Given that get_value() is central to serializer input processing,
any reordering of conditionals should be validated carefully.

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3. Test Coverage

The added regression test correctly validates:

• Indexed keys (colors[0], colors[1])
• partial=True
• Preservation of ordering

However, additional coverage would strengthen confidence:

Suggested additional tests

• partial=True with non-indexed list input:

  data = MultiValueDict({'colors': ['#ffffff', '#000000']})