RFR: 8319339: Internal error on spurious markup in a hybrid snippet

[Pavel Rappo]

Please review this PR to fix an internal error caused by (malfunctioning diagnostics for) spurious markup in hybrid and external snippets.

Spurious markup resembles snippet markup, but isn't one. In particular, spurious markup has no effect on snippet processing and, if appears in the displayed region, is shown as is.

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Commit messages:
 - fix
 - test
 - refactor and strengthen the existing test
 - refactor: clarify comment

Changes: https://git.openjdk.org/jdk/pull/16501/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16501&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319339
  Stats: 142 lines in 3 files changed: 133 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/16501.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16501/head:pull/16501

PR: https://git.openjdk.org/jdk/pull/16501