RFR: 8358015: Fix SequencedMap sequenced view method specifications [v2]

[Stuart Marks]

> For a full explanation, see the bug report [JDK-8358015](https://bugs.openjdk.org/browse/JDK-8358015).
> 
> This PR includes three related changes:
> * New overrides in SequencedMap view collection implementations, which improve their behavior.
> * Update to `@implSpec` clauses to reflect this behavior (which was previously ill-specified).
> * Update to a test which relied on unspecified behavior.

Stuart Marks has updated the pull request incrementally with one additional commit since the last revision:

  Tweak nsee() declaration; adjust specs per suggestion; update copyrights.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/25515/files
  - new: https://git.openjdk.org/jdk/pull/25515/files/ee8f784e..ff9fe9ab

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25515&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25515&range=00-01

  Stats: 9 lines in 3 files changed: 0 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/25515.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25515/head:pull/25515

PR: https://git.openjdk.org/jdk/pull/25515