Fix TreeNodeFilter OR-pattern diagnostics

[Evangelink]

Description

• Fixes https://github.com/--treenode-filter pattern matching is at least non obvious, potentially errored #7300 by clarifying behavior and improving failure diagnostics for unsupported parenthesized full-path OR patterns.
• Root cause: Tree-node OR is valid within a single path segment, but patterns like (/A/B/C/X)|(/A/B/C/Y) hit separator processing inside parentheses and throw a generic invalid operation.
• Changes:
  • Add regression tests for supported single-segment OR and exact-match behavior in TreeNodeFilterTests.cs.
  • Improve parser exception guidance and grammar remarks in TreeNodeFilter.cs.
  • Add changelog entry in Changelog-Platform.md.

Validation

• Ran: dotnet test Microsoft.Testing.Platform.UnitTests.csproj -c Debug --filter "FullyQualifiedName~TreeNodeFilterTests"
• Result: 138/138 passed (net9.0, net8.0, net462).

Notes

This change does not expand grammar to support full-path OR in parentheses; it adds explicit guidance and test coverage for current supported syntax.

[Youssef1313]

I have doubts that the problem here is clarity of error message. The behavior seems buggy IMO. Let's discuss offline.