Harden active notification button indexing

[AI-DEV-BOT]

Description of Change

Fix active notification button index assignment in NotificationActiveStyle.AddButtonAction().

When a ButtonAction is added with ButtonIndex.None, the method now assigns the first unused supported button slot from ButtonIndex.First through ButtonIndex.Third instead of deriving the index from buttonDictionary.Count. This avoids invalid indexes after three buttons and prevents duplicate-key failures when buttons have been removed.

Explicit button indexes are also validated so values outside the supported active notification button range fail with NotificationError.InvalidParameter.

Validation:

• git diff --check

API Changes

• ACR: N/A

None.

[gemini-code-assist]

Code Review

This pull request improves the button index assignment and validation logic in NotificationActiveStyle.cs by searching for the first available index within the valid range (First to Third) when no index is specified, and throwing appropriate exceptions for invalid or overflowing indices. There are no review comments, and I have no additional feedback to provide.

[JoonghyunCho]

🤖 [AI Review]

Reviewed — no findings.

Scope checked:

• Auto-assign path (button.Index == ButtonIndex.None) replaces (ButtonIndex)buttonDictionary.Count with a scan over First..Third for the first unused slot — correctly handles the post-remove gap case the old code mis-indexed.
• New explicit-index validation upper-bounds at ButtonIndex.Third; values outside First..Third (previously silently inserted) now throw — consistent with ButtonIndex enum (None=-1, First=0, Second=1, Third=2, max 3 slots).
• Both new throw sites use NotificationError.InvalidParameter, which NotificationErrorFactory.GetException maps to ArgumentException — already covered by the existing <exception cref="ArgumentException"> doc tag, so no new XML tag required.
• Public method signature unchanged (public void AddButtonAction(ButtonAction)); existing <summary>, <param>, <since_tizen> remain accurate.
• Loop termination is guaranteed: at most 3 iterations (First..Third inclusive), and return exits as soon as a slot is found.

No 🔴 critical issues, no 🟡 suggestions to flag.

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