fix: accept plugin_unique_identifier from query params for DecodePluginFromIdentifier (#593)

Changed the struct tag from `json:` to `form:` for PluginUniqueIdentifier field
in DecodePluginFromIdentifier handler. This allows the parameter to be bound
from query string instead of JSON body.

This is consistent with similar GET endpoints (FetchPluginManifest,
FetchPluginFromIdentifier) which already use `form:` tags.

The change is needed because:
- GET requests with body are rejected by some HTTP intermediaries (e.g., Cloud Run)
- HTTP standards state that GET request body has no defined semantics
- Query parameters are the appropriate way to pass data in GET requests

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: minodisk

internal/server/controllers/plugins.go

@@ -166,7 +166,7 @@ func ReinstallPluginFromIdentifier(app *app.Config) gin.HandlerFunc {
 func DecodePluginFromIdentifier(app *app.Config) gin.HandlerFunc {
 	return func(c *gin.Context) {
 		BindRequest(c, func(request struct {
-			PluginUniqueIdentifier plugin_entities.PluginUniqueIdentifier `json:"plugin_unique_identifier" validate:"required,plugin_unique_identifier"`
+			PluginUniqueIdentifier plugin_entities.PluginUniqueIdentifier `form:"plugin_unique_identifier" validate:"required,plugin_unique_identifier"`
 		}) {
 			c.JSON(http.StatusOK, service.DecodePluginFromIdentifier(app, request.PluginUniqueIdentifier))
 		})