fix(driver-ablegacy): resolve Medium code-review finding (Driver.AbLegacy-003)

TryParse now rejects three classes of malformed PCCC address:
- Sub-element + bit-index together (e.g. T4:0.ACC/2) — never valid in PCCC
- File number on I/O/S system files (e.g. I3:0, S2:1) — single-letter only
- Sub-element on non-T/C/R files (e.g. B3:0.DN, N7:0.FOO) — only Timer,
  Counter, and Control files carry structured elements

New helper predicates IsNoFileNumberLetter / IsSubElementFileLetter
keep the parser's intent clear. Regression tests added in AbLegacyAddressTests.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyAddressTests.cs

@@ -65,4 +65,41 @@ public sealed class AbLegacyAddressTests
         a.ShouldNotBeNull();
         a.ToLibplctagName().ShouldBe(input);
     }
+
+    // ---- Driver.AbLegacy-003: Parser tightening ----
+
+    [Theory]
+    [InlineData("T4:0.ACC/2")]      // sub-element + bit index — never valid in PCCC
+    [InlineData("C5:0.PRE/3")]
+    public void TryParse_rejects_subelement_plus_bitindex(string input) =>
+        AbLegacyAddress.TryParse(input).ShouldBeNull();
+
+    [Theory]
+    [InlineData("I3:0")]            // I is a system file — no file number allowed
+    [InlineData("O2:1")]
+    [InlineData("S2:1")]
+    public void TryParse_rejects_file_number_on_IOS_files(string input) =>
+        AbLegacyAddress.TryParse(input).ShouldBeNull();
+
+    [Theory]
+    [InlineData("B3:0.DN")]         // B (bit) file has no structured elements
+    [InlineData("N7:0.FOO")]        // N (integer) file has no structured elements
+    [InlineData("F8:0.ACC")]        // F (float) file has no structured elements
+    [InlineData("L9:0.PRE")]        // L (long) file has no structured elements
+    public void TryParse_rejects_subelement_on_non_structured_file(string input) =>
+        AbLegacyAddress.TryParse(input).ShouldBeNull();
+
+    [Theory]
+    [InlineData("T4:0.ACC")]        // T, C, R are the only structured-element files
+    [InlineData("C5:0.PRE")]
+    [InlineData("R6:0.LEN")]
+    public void TryParse_accepts_subelement_only_on_TCR_files(string input) =>
+        AbLegacyAddress.TryParse(input).ShouldNotBeNull();
+
+    [Theory]
+    [InlineData("I:0/0")]           // I/O/S without file number are valid
+    [InlineData("O:1/2")]
+    [InlineData("S:1")]
+    public void TryParse_accepts_IOS_without_file_number(string input) =>
+        AbLegacyAddress.TryParse(input).ShouldNotBeNull();
 }