feat: add showToolbarOnTap option for iOS single-tap context menu

[futaba3]

Description

Adds showToolbarOnTap parameter to enable the standard iOS single-tap context menu behavior (Paste, Select All, etc.) when opted in.

Closes #235

Motivation

As described in #235, on iOS, tapping an already-focused Pinput does not show the context menu — only a long press triggers it. The standard Flutter TextField toggles the context menu on a single tap when the field is already focused, so this PR adds an opt-in parameter to bring that behavior to Pinput.

I opted for a new parameter rather than changing the default behavior, since the current behavior may be intentional (e.g. to avoid accidental toolbar appearance during the initial focus transition). If there is a reason the existing behavior is by design, I'd love to hear it — happy to adjust the approach accordingly.

Implementation notes

Simply removing hideToolbar() from onSingleTapUp was not sufficient due to two constraints I ran into:

1. _handleSelectionChanged rewrites the selection on every tap, so the framework's toggleToolbar condition (previousSelection == currentSelection) always resolves to hideToolbar rather than toggling.
2. rendererIgnoresPointer: true leaves _selectionOverlay as null, so showToolbar() does nothing in this case — toggleToolbar() is needed because it creates the overlay when null.

As a workaround, toolbar visibility is tracked with a manual _isToolbarVisible flag, since EditableTextState.toolbarIsVisible is not accessible from outside the framework.

Testing

• Verified on iOS simulator (Flutter 3.41.4, iOS 26.2)
• Verified on Android device (Flutter 3.41.4, Android13) — behavior unchanged
• showToolbarOnTap: false (default): long-press-only behavior unchanged
• showToolbarOnTap: true: 1st tap focuses, 2nd tap shows menu, 3rd tap hides menu
• Added widget tests for iOS and Android variants
• Note: actual context menu rendering is OS-level UI and cannot be verified in widget tests

Demo (showToolbarOnTap: true, iOS)

The demo was recorded with showToolbarOnTap: true set in the example app for
verification purposes. This parameter is not changed in the example app in this PR.

	before	after
iOS	CleanShot.2026-06-26.at.15.21.09.mp4	CleanShot.2026-06-26.at.14.54.32.mp4
Android	XRecorder_26062026_150152.mp4	Behavior unchanged

Breaking Changes

None — showToolbarOnTap defaults to false, preserving existing behavior.

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This is my first contribution to an open source project. If anything looks off — code style, approach, or anything else — please let me know and I'll be happy to update it.