RFR: 8365086: CookieStore.getURIs() and get(URI) should return an immutable List [v2]

[Jaikiran Pai]

> Can I please get a review of this change which proposes to address the issue noted in https://bugs.openjdk.org/browse/JDK-8365086?
> 
> `java.net.CookieStore` has some APIs which return a `java.util.List`. These API are specified to return an immutable `List`. The JDK ships an implementation of the `CookieStore` - the `java.net.InMemoryCookieStore`. The implementations of the `getURIs()` and `get(URI)` methods  in the `InMemoryCookieStore` currently return a `List` which is modifiable.
> 
> The changes in this PR fix those two method to return an immutable `List` to match the specification. A new regression test has been introduced to reproduce the issue and verify the fix.
> 
> I think this may not require a CSR since this fixes the implementation to match the already existing specification in `CookieStore`. I will create one if anyone suggests we should.

Jaikiran Pai has updated the pull request incrementally with one additional commit since the last revision:

  Volkan's review - refactor assertions into a method

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/26698/files
  - new: https://git.openjdk.org/jdk/pull/26698/files/914fde9a..7b92b0a6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26698&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26698&range=00-01

  Stats: 21 lines in 1 file changed: 6 ins; 9 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/26698.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26698/head:pull/26698

PR: https://git.openjdk.org/jdk/pull/26698