Clarify chained configuration TryGet semantics and apply review-thread fixes

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main PR

Description

This updates only the code requested in review thread #pullrequestreview-4799878306: clarify ChainedConfigurationProvider.TryGet semantics and align tests with nullable contracts.

• Provider behavior + API docs

  • TryGet now treats any non-null value as present (including empty string).
  • XML docs now explicitly state non-null semantics.

• Tests aligned with contract

  • Added focused coverage for:
    • empty string value returns true
    • missing key returns false and null
  • TryGet out usage is nullable (string?) to match IConfigurationProvider.TryGet.

public bool TryGet(string key, out string? value)
{
    value = _config[key];
    return value is not null;
}

Customer Impact

Without this change, chained configuration can incorrectly treat empty string as missing, allowing lower-priority values to win unexpectedly. API docs also remain ambiguous for consumers interpreting TryGet.

Regression

This is a behavioral correctness fix for chained provider precedence with empty-string values; null remains treated as absence.

Testing

Targeted unit tests were added for TryGet empty-string and missing-key behavior, matching the intended nullable contract.

Risk

Low. The functional change is a single-condition update in TryGet, with scoped tests covering the affected behavior.

Package authoring no longer needed in .NET 9

IMPORTANT: Starting with .NET 9, you no longer need to edit a NuGet package's csproj to enable building and bump the version.
Keep in mind that we still need package authoring in .NET 8 and older versions.

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