fix: prevent tag removal via backspace when readOnly is enabled

[naaa760]

#8066

Problem

The TagsInput component with readOnly={true} was incorrectly allowing users to remove tags by pressing backspace when the input field was focused. This violated the expected read-only behavior where no modifications should be possible.

Root Cause

Multiple input event handlers were missing readOnly property checks, allowing tag modifications even when the component was supposed to be read-only.

Solution

Added readOnly checks to all input handlers that can modify tags:

• ✅ Backspace key (tag removal)
• ✅ Enter key (tag addition)
• ✅ Split characters like comma (tag addition)
• ✅ Paste events (tag addition)
• ✅ Blur with acceptValueOnBlur (tag addition)
• ✅ Dropdown option selection (tag addition)

Testing

• Added 6 comprehensive test cases covering all input scenarios
• All existing tests continue to pass (62/62)
• Verified behavior matches other Mantine read-only components