[FLINK-40452] Cap exception fetching by size instead of line count

[mbalassi]

JobStatusObserver capped JobManager exception text in K8s events byline count (stacktrace.split("\n"), keep first N lines). A single unbroken line of any size produces one array element, so it bypassed the cap entirely and could fill the operator heap.

Approach

• Replaced events.exceptions.stacktrace-lines (line count, default 5)
  with events.exceptions.stacktrace.max.length (character count, default
  2048 — matches the two existing sibling options
  exception.stacktrace.max.length / exception.field.max.length; no
  config in the repo defaults to 4096, so 2048 was kept rather than
  raised).
• Replaced the truncation itself: stacktrace.length() >
  maxStackTraceLength + bounded substring, instead of splitting on \n.
  Also removes an unbounded 3-4x string-copy chain downstream of the old
  cap.
• Old option removed outright (no value-preserving migration exists
  between line-count and character-count semantics).
• Added a regression test (5000-char single line, no newlines) and
  updated existing tests, generated config docs, and hand-written
  event-format docs (both locales).

Testing

Updated existing unit test and added a new test to explicitly cover the bug scenario reported in FlINK-40452.

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Generated-by: Claude Code (Opus 5)

[Dennis-Mircea]

I also took a closer look at JobStatusObserver and I see that the recordJobErrorIfPresent method is not covered by this feature, and currently it can emit event messages of any size. I'd say will great to extend the max length support for this method as well.

Additionally, the observeJobManagerExceptions has a TODO in place as of now which brings into light another hot spot for this max cap feature. Currently, the history of exceptions is being retrieved via REST and can have a big size as well, so I'd say it is worth considering fixing that place as well and to wrap everything at once for JobStatusObserver.

[Dennis-Mircea]

Wouldn't be better/safer to first deprecate this option that was introduced as part of 1.12.0 operator release and remove it in further releases?

[Dennis-Mircea]

I'd say it's good to add a defensive support for negative values, and maybe consider -1 value as unlimited length.

[spuru9]

LGTM post the addressing the comments from Dennis.

@mbalassi Can you take a look at them.

[spuru9]

nit optional: we can simply this to

          if (stacktrace != null && !stacktrace.isBlank()) {                                                                                                                                                     
              getSubstringWithMaxLength(stacktrace, maxStackTraceLength).ifPresent(eventMessage::append);                                                                                                        
              if (stacktrace.length() > maxStackTraceLength) {                                                                                                                                                   
                  eventMessage                                                                                                                                                                                   
                          .append("... (")                                                                                                                                                                       
                          .append(stacktrace.length() - maxStackTraceLength)                                                                                                                                     
                          .append(" more characters)");                                                                                                                                                          
              }                                                                                                                                                                                                  
          }