Bind immutable collections and maps by default

[ABin-Huang]

Summary

When binding to collection or map interface types (List, Set, SortedSet, Map, SortedMap), the bound result is now wrapped in an unmodifiable collection or map. This prevents accidental mutation of bound configuration state and aligns with the immutable configuration properties approach discussed in the issue.

Design decisions

1. Interface types → immutable by default: Properties declared as List, Set, SortedSet, Map, or SortedMap receive unmodifiable instances.

2. Concrete types → remain mutable: Properties declared as concrete mutable types (e.g. LinkedList, ArrayList, HashMap, TreeMap) remain mutable. The user has explicitly chosen a mutable implementation, and creating immutable subclasses for every concrete type is not practical.

3. Existing values → preserve mutability: When an existing value is supplied via Bindable.withExistingValue(), its mutability is preserved. The user has provided their own instance and may rely on its mutability.

4. Implementation location: The immutability wrapping happens in Binder.handleBindResult() after the final type conversion, ensuring all aggregate types (including Set which is converted from the internal List) are correctly handled.

Changes

• Binder.java: Added makeImmutableIfNeeded() and helper methods to wrap bound collections/maps in unmodifiable instances when the target type is an interface.
• CollectionBinderTests.java: Added tests for unmodifiable List, Set, concrete type mutability, and nested collections.
• MapBinderTests.java: Added tests for unmodifiable Map and concrete type mutability.

Notes for reviewers

• This is a behavioral change: code that mutates bound collections/maps declared as interface types will now throw UnsupportedOperationException. Users who need mutable collections should declare concrete types or provide an existing mutable value.
• An opt-out mechanism (e.g. a @ConfigurationProperties attribute or system property) can be added if the team feels it's needed. I wanted to keep this PR focused on the core behavior change and get feedback on the approach first.

Closes gh-27582