Ignite 28747 quick fix

[EgorBaranovEnjoysTyping]

IGNITE-28747: Fix NPE in GridToStringBuilder#handleRecursion

Problem

NullPointerException occurs when GridToStringBuilder.handleRecursion() is called during toString()
building of recursive structures. The tail field in SBLimitedLength remains null because its
initialization was only triggered inside onWrite(), but handleRecursion() directly calls buf.a(...)
methods, bypassing onWrite().

Root Cause

In SBLimitedLength, the original code in master branch directly called tail.append(...) in all a(...)
methods without checking if tail is initialized first. This caused NPE when recursion occurred.

Solution

Added centralized initTailIfAbsent() method that checks and creates tail only when needed. Replaced direct
tail.append(...) calls with initTailIfAbsent() + tail.append(...) pattern.

Related

JIRA: https://issues.apache.org/jira/browse/IGNITE-28747

[wernerdv]

Suggested change

	SBLimitedLength sbLimitedLength = new SBLimitedLength(0);
	SBLimitedLength sb = new SBLimitedLength(0);