Address code review feedback: fix resource disposal, unused imports, and code patterns

[Copilot]

Applies 26 code review comments from PR #136 covering resource management, code style, and efficiency issues across C#, Python, and TypeScript samples.

C# Changes

• Resource disposal: Wrap HttpClient instances in using statements in AspNetExtensions.cs
• Foreach filtering: Use explicit .Where() for clarity in MyAgent.cs and AspNetExtensions.cs
• Code patterns:
  • Replace ContainsKey + indexer with TryGetValue
  • Use StringBuilder for string concatenation in loops
  • Simplify conditional expressions with ternary operators
  • Remove redundant null checks and unused assignments

Python Changes

• Unused imports: Remove asyncio, setup_observability, and os imports in agent files
• Dead code: Remove unnecessary pass statements

TypeScript Changes

• Unused imports: Remove ToolRunner import in chatFlowService.ts

Script Changes

• Unused variables: Remove unused comment variable in assign_points.py

Example fix in AspNetExtensions.cs:

// Before: implicit filtering
foreach (var item in collection)
{
    if (item != null) { ... }
}

// After: explicit filtering
foreach (var item in collection.Where(x => x != null))
{
    ...
}

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