diagnostics: Fix specificity calculation for message-based matches

Fixed a bug in `calculate_match_specificity` where the priority bonus
for message-based pattern matches was computed but not returned.
The function was executing `specificity + 2` instead of
`specificity += 2`, resulting in message-based matches having the same
priority as code-based matches.

This caused the diagnostic configuration system to incorrectly handle
cases where both code-based and message-based patterns matched the same
diagnostic. Message-based patterns should have higher priority
(specificity +2) to allow more fine-grained control.

Added a test case to verify that message-based literal matches
(specificity=4) are correctly prioritized over code-based regex matches
(specificity=1).

Author: aviatesk

src/diagnostics.jl

@@ -148,7 +148,7 @@ function calculate_match_specificity(
     end
     specificity == 0 && return specificity
     if is_message_match
-        specificity + 2
+        specificity += 2
     end
     return specificity
 end

test/test_diagnostics.jl

@@ -562,6 +562,32 @@ end
             JETLS.apply_diagnostic_config!(diagnostics, manager)
             @test isempty(diagnostics)
         end
+
+        # message-based patterns should have higher priority than code-based patterns
+        let diagnostics = [
+                make_test_diagnostic(;
+                    code = JETLS.LOWERING_MACRO_EXPANSION_ERROR_CODE,
+                    severity = DiagnosticSeverity.Error,
+                    message = "Macro name `@interface` not found")
+            ]
+            manager = make_test_manager(Dict{String,Any}(
+                "diagnostic" => Dict{String,Any}(
+                    "patterns" => [
+                        Dict{String,Any}(
+                            "pattern" => "lowering/macro-expansion-error",
+                            "match_by" => "code",
+                            "match_type" => "literal",
+                            "severity" => "hint"),
+                        Dict{String,Any}(
+                            "pattern" => "Macro name `@interface` not found",
+                            "match_by" => "message",
+                            "match_type" => "literal",
+                            "severity" => "info"),
+                    ])))
+            JETLS.apply_diagnostic_config!(diagnostics, manager)
+            @test length(diagnostics) == 1
+            @test only(diagnostics).severity == DiagnosticSeverity.Information
+        end
     end
 end