fix: return clear error for duplicate plugin names (fix #13798)

[akuligowski9]

Summary

• Adds a PluginNameAlreadyExists(String) error variant (consistent with WindowLabelAlreadyExists / WebviewLabelAlreadyExists)
• PluginStore::register() now returns Result<()> and rejects duplicates
• AppHandle::plugin() checks for duplicates before running initialize(), then propagates the error
• Builder::plugin() warns via log::warn! and replaces the old plugin (preserves backward compat since it returns Self)

Motivation

When a user registers two plugins with the same name, the duplicate silently replaces the original. If the replaced plugin was a core plugin (e.g. "path", "event", "window"), the app later panics with an inscrutable state() called before manage() — giving zero indication that a duplicate plugin name was the root cause.

How it works

Scenario	Behavior
Two plugins with the same name on Builder	log::warn!, second replaces first (backward compat)
User plugin collides with core plugin name	register_core_plugins() → Err(PluginNameAlreadyExists("window")) — clear error
Runtime duplicate via AppHandle::plugin()	Err(PluginNameAlreadyExists("my-plugin")) — caller can handle it

Test plan

• cargo check -p tauri compiles
• cargo clippy -p tauri -- -D warnings passes
• cargo test -p tauri — all 52 unit tests + 86 doc-tests pass

🤖 Generated with Claude Code

[FabianLars]

Thanks for the PR! Would you mind signing (and force-pushing) your commit? Our branch protection rules require that :)

[FabianLars]

I don't think it makes sense to have the very same check both here and in the 2 register() methods above

[akuligowski9]

Good point — I've moved the duplicate check to Builder::plugin() only, so it catches mistakes early at app setup. Runtime registration via AppHandle::plugin() keeps the existing replace behavior, so devs still have the choice to handle duplicates themselves.

[FabianLars]

not the biggest fan of the behavior change. i think devs should handle that themselves so they have a choice

[akuligowski9]

Agreed — I've updated the approach. The duplicate check now only applies at build time in Builder::plugin(), where it's clearly a developer mistake (registering the same plugin twice in setup code). Runtime registration via AppHandle::plugin() keeps the existing silent-replace behavior, so devs retain full control.

[github-actions]

Package Changes Through 82b277a

There are 4 changes which include tauri-cli with patch, @tauri-apps/cli with patch, tauri-bundler with patch, tauri with patch

Planned Package Versions

The following package releases are the planned based on the context of changes in this pull request.

package	current	next
tauri-bundler	2.8.0	2.8.1
tauri	2.10.2	2.10.3
@tauri-apps/cli	2.10.0	2.10.1
tauri-cli	2.10.0	2.10.1

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[akuligowski9]

Hi! Just a gentle ping — this PR adds a clear error message for duplicate plugin names (resolves #13798) and has been ready for review since February. Let me know if there's anything I should adjust!