Make precedence source ordered

[linglingye001]

Why this PR?

Fix #621

ConfigurationFeatureDefinitionProvider always evaluates the Microsoft schema first, source order is never consulted when the same flag is split across schemas. A Microsoft-schema definition from the first provider beats a .NET-schema definition from every later provider.

Visible Changes

• Lookup visits sources from highest to lowest precedence, checking Microsoft schema before .NET within each source.
• Custom merging now selects the whole definition from one source for both schemas.
• Preserved default behavior, case handling, deduplication, chaining, and root fallback.
• Deduplicates cross-schema entries during feature enumeration.

Example

When custom merging is enabled:

Provider` 1: Microsoft schema → MyFlag = false
Provider 2: .NET schema → MyFlag = targeting filter

Winning schema map:
MyFlag → .NET

Result: Provider 2 wins and the targeting filter is parsed.