fix: keep HttpHeaders value lists immutable

[audmartina9]

HttpHeaders.of currently exposes its copied mutable value lists through allValues and map. A caller can therefore change an existing header after construction, contrary to the JDK's immutable HttpHeaders contract. Wrap each copied list with Collections.unmodifiableList before storing it.

The regression tests cover add, set, iterator removal, clearing a list obtained from map, and isolation from subsequent changes to the caller's input list.

Validation on Scala 3.3.7 / Scala Native 0.5.12 with JDK 25:

• Original production source with the regression tests: native suite 23/25 passed; the two immutability cases failed.
• Fixed source: native suite 25/25 passed.
• JDK 25 reference: JVM suite 25/25 passed.

Commands (focused HttpHeaders suite, not all project test suites):

./mill --no-server modules.java-http-client.test.testOnly snhttp.test.java.net.http.HttpHeadersTest
./mill --no-server modules.java-http-client.jvm.test.testOnly snhttp.test.java.net.http.HttpHeadersTest

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[lqhuang]

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