fix: Set Kusto wait strategy encoding to UTF-8

[MattKotsenas]

What does this PR do?

The Kusto wait strategy uses the default encoding. For .NET Framework, this resolves to Windows-1252, which causes the Kusto emulator to return a 415 Media Type not Supported error. In .NET Core, it is UTF-8.

This allows .NET Framework tests to run by setting the encoding to always be UTF-8.

Why is it important?

Required to use Testcontainers in a .NET Framework app. Otherwise, the Testcontainer waits forever.

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[coderabbitai]

Summary by CodeRabbit

• Bug Fixes
  • Fixed character encoding handling for Kusto emulator requests to ensure proper data transmission and compatibility.

Walkthrough

The Kusto emulator HTTP request encoding was changed from the platform default (Encoding.Default) to UTF-8 for the JSON payload used in POST requests; the JSON content itself and all other logic remain unchanged.

Changes

Cohort / File(s)	Summary
HTTP request encoding standardization src/Testcontainers.Kusto/KustoBuilder.cs	Replaced Encoding.Default with Encoding.UTF8 when creating StringContent for Kusto emulator POST requests. No other logic or public signatures changed.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

• Single-line, single-file change limited to encoding parameter.
• Files to spot-check: src/Testcontainers.Kusto/KustoBuilder.cs.

Poem

🐰 In bytes I hop from default to UTF-8,

The JSON travels tidy, uniform and neat,
No more surprises from platform skies,
A smaller jump, yet ever wise —
🥕 code snug, and tests complete.

Pre-merge checks and finishing touches

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description check	✅ Passed	The PR description covers the mandatory sections (What and Why) with clear technical context about the encoding issue and its impact on .NET Framework compatibility.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Title check	✅ Passed	The title clearly and concisely summarizes the main change: fixing the Kusto wait strategy to use UTF-8 encoding instead of default encoding, which directly addresses the issue described in the PR objectives.

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[HofmeisterAn]

Thanks! That's something we should update across the whole repo.