CSHARP-6206: KMS master key exposure via unredacted credential serialization in driver settings string (port to main branch)

[papafe]

https://jira.mongodb.org/browse/CSHARP-6206

[Copilot]

🟢 Approval recommended

The change is narrowly scoped to string serialization, includes targeted test updates/additions, and effectively prevents credential exposure without affecting operational encryption behavior.

Pull request overview

This PR addresses a security issue in the C# driver’s settings string representation by ensuring KMS provider credentials are not serialized in AutoEncryptionOptions.ToString() (which is surfaced via MongoClientSettings.ToString()), preventing accidental leakage of sensitive key material into logs/telemetry.

Changes:

• Redacts all KMS provider option values in AutoEncryptionOptions.ToString() while retaining provider names.
• Updates the existing ToString_should_return_expected_result test to match the redacted output.
• Adds tests to verify KMS provider credentials are not exposed and that empty kmsProviders are rendered correctly.

File summaries

File	Description
src/MongoDB.Driver/AutoEncryptionOptions.cs	Changes ToString() to emit KmsProviders as { "<provider>" : "<hidden>" } for each provider, preventing credential/key leakage.
tests/MongoDB.Driver.Tests/Encryption/AutoEncryptionOptionsTests.cs	Updates/extends ToString() assertions to validate redaction behavior and empty-provider formatting.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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