.NET: Validate SkillsInstructionPrompt contains {0} placeholder in FileAgentSkillsProvider (#4642)

* Fix FileAgentSkillsProvider accepting SkillsInstructionPrompt without {0} placeholder (#4638)

BuildSkillsInstructionPrompt validated only format-string syntax via
string.Format(template, ""), which silently accepted templates without a
{0} placeholder. The generated skills list was then dropped from the final
instructions.

Tighten validation to format with a sentinel string and verify it appears
in the output, rejecting templates that do not reference argument 0 with
an ArgumentException.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix netstandard2.0 compat and simplify prompt template validation (#4638)

- Replace string.Contains(string, StringComparison) with IndexOf for
  netstandard2.0/net472 compatibility
- Remove sentinel round-trip check; validate {0} directly on the raw
  template string using IndexOf
- Add positive test verifying custom SkillsInstructionPrompt with {0}
  is accepted and applied to output

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <copilot@github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

dotnet/src/Microsoft.Agents.AI/Skills/FileAgentSkillsProvider.cs

@@ -175,15 +175,23 @@ public sealed partial class FileAgentSkillsProvider : AIContextProvider
             try
             {
                 _ = string.Format(optionsInstructions, string.Empty);
-                promptTemplate = optionsInstructions;
             }
             catch (FormatException ex)
             {
                 throw new ArgumentException(
-                    "The provided SkillsInstructionPrompt is not a valid format string. It must contain a '{0}' placeholder and escape any literal '{' or '}' by doubling them ('{{' or '}}').",
+                    "The provided SkillsInstructionPrompt is not a valid format string.",
                     nameof(options),
                     ex);
             }
+
+            if (optionsInstructions.IndexOf("{0}", StringComparison.Ordinal) < 0)
+            {
+                throw new ArgumentException(
+                    "The provided SkillsInstructionPrompt must contain a '{0}' placeholder for the generated skills list.",
+                    nameof(options));
+            }
+
+            promptTemplate = optionsInstructions;
         }
 
         if (skills.Count == 0)