fix: guard against null ITextEditor document provider

[dungdong-aws]

Problem

Fixes #510. An editor that implements ITextEditor but returns null from
getDocumentProvider() makes Amazon Q raise

Cannot invoke "IDocumentProvider.getDocument(Object)" because the return value of
"ITextEditor.getDocumentProvider()" is null

The reporter sees the workbench's generic "An error has occurred. See error log for
more details." dialog simply by activating such an editor — no Amazon Q action is
required.

Root cause

IDocumentProvider is an optional part of the Eclipse text-editor API;
ITextEditor.getDocumentProvider() is allowed to return null, and third party
editors do. AutoTriggerPartListener.partActivated dereferences it unconditionally
for every activated ITextEditor:

var document = editor.getDocumentProvider().getDocument(editor.getEditorInput());
document.addDocumentListener(docListener);

Because partActivated is an IPartListener2 callback, the NPE escapes into the
workbench, which reports it through its status handler — hence the dialog on every
activation. onStart() in the same class already null-checked the document but
not the provider.

The repository already treats this API as nullable elsewhere — AbapUtil (~221),
AmazonQLspClientImpl (~505) and InlineChatSession (~109) all guard it — so this
change applies the existing in-repo convention to the paths that still miss it.

Changes

Production (~50 lines):

• AutoTriggerPartListener: added a private resolveDocument(ITextEditor) helper
  that returns null when the editor, its provider, or its document is null.
  partActivated now returns early instead of attaching a listener, and onStart
  reuses the helper so a null provider re-arms the existing 1-second poll exactly
  as a null document already did.
• QEclipseEditorUtils: added an equivalent private getDocument(ITextEditor)
  helper and routed getSelectionRange and insertAtCursor through it; both now
  return Optional.empty() rather than throwing. These are the send-to-chat and
  insert-at-cursor paths, reachable with the same editor.

Behaviour for editors that do have a document provider is unchanged.

The guards are deliberately local to each class rather than shared through a public
QEclipseEditorUtils helper: QEclipseEditorUtils is replaced by mockStatic in
four existing test classes, so routing AutoTriggerPartListener through it would
make the guard unobservable in that class's own unit tests.

Remaining call sites (intentionally not in this PR)

The same unchecked dereference exists in the inline-chat and agentic-edit paths —
InlineChatDiffManager (99, 100, 198, 199, 244), InlineChatUIManager (425) and
AmazonQLspClientImpl (398, 400). Those are not part of this fix because they sit
behind InlineChatSession.startSession, which already wraps the call in
try/catch and ends the session, so they produce a handled failure rather than the
reported workbench dialog. Keeping this PR to the unhandled path keeps it atomic
and reviewable; the inline-chat sites are worth a separate follow-up.

Edge cases covered

• getDocumentProvider() returns null → activation ignored, no listener attached,
  no active document recorded, and the subsequent partDeactivated is also a no-op.
• Provider present but getDocument(input) returns null → same no-op.
• onStart with a null provider → retries on the existing timer instead of throwing
  on the UI thread.
• Editor itself null (already possible via getActiveTextEditor()) → handled by
  the same helper.

Tests / validation

• Added AutoTriggerPartListenerTest#testPartActivateWithNullDocumentProvider and
  #testPartActivateWithNullDocument, both asserting no throw and that no document
  listener is attached or detached.
• Not run locally. This environment has no Maven/Tycho toolchain and cannot
  enforce a memory cgroup cap, so no build or test execution was performed. CI on
  this PR (mvn -B package, which also runs checkstyle with
  includeTestSourceDirectory) is the first full validation.
• Not reproduced at runtime. No Eclipse instance was run and no custom
  provider-less ITextEditor was built. The mechanism is established from source
  and from the exact exception message on the issue; the throw site is inferred from
  the activation-only reproduction, since the issue quotes the message without a
  stack trace.

Release target

Base main (repository default; the Maven workflow runs for PRs targeting main).

Ticket

• GitHub issue: When an editor in Eclipse is an ITextEditor instance without DocumentProvider, an error message is thrown. #510