fix(button): apply first-time style patches and clear stale style internals

[ndonkoHenri]

Fix #6118

Description

When a Button starts without bgcolor, the first click (None -> GREEN) could update Python state but not the UI.
Also, after changing style props back to defaults, stale style data could remain and keep old visuals.

Root cause

1. Dart patch apply path

   • First-time style updates arrive as an ADD patch to _internals.style.
   • If _internals didn’t exist yet, patch target resolution failed to materialize missing intermediate maps.

2. Python control internals

   • Button / IconButton wrote _internals["style"] when style-related props were set,
   • but didn’t clear it when props returned to defaults.

Changes

• Dart

  • Enhanced getPatchTarget(...) with optional createMissingMaps.
  • ADD operations now resolve targets with createMissingMaps: true, allowing creation of missing intermediate maps (e.g. _internals) before inserting new keys.

• Python

  • Clear stale style internals when style-driving values are default:
    • self._internals.pop("style", None)

Summary by Sourcery

Ensure button style patches correctly materialize intermediate state and clear stale style metadata.

Bug Fixes:

• Fix JSON patch ADD operations so missing intermediate maps (e.g., control internals) are created before inserting new entries, ensuring initial style updates apply to controls like Button.
• Clear stale style internals for Button and IconButton when style-driving properties return to default values so visuals revert correctly.

Tests:

• Add a control patching test to verify ADD operations create missing intermediate maps in control properties.

[sourcery-ai]

We've reviewed this pull request using the Sourcery rules engine

[Copilot]

Pull request overview

This PR fixes button style update issues where first-time style changes (e.g., None -> GREEN) failed to appear in the UI, and stale style data persisted after properties returned to defaults.

Changes:

• Enhanced Dart patch resolution to create missing intermediate maps (like _internals) when applying ADD operations
• Modified Python Button and IconButton controls to clear stale style internals when style properties reset to defaults

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
sdk/python/packages/flet/src/flet/controls/material/button.py	Clears _internals["style"] when style properties return to defaults
sdk/python/packages/flet/src/flet/controls/material/icon_button.py	Clears _internals["style"] when style properties return to defaults
packages/flet/lib/src/models/control.dart	Adds createMissingMaps parameter to getPatchTarget to materialize missing intermediate maps during ADD operations
packages/flet/test/utils/control_test.dart	Adds test case verifying ADD patch creates missing intermediate map

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