IGNITE-28310 Optimize ClientInboundMessageHandler#writeError

[ptupitsyn]

• Remove legacy stack trace retrieval code, use simple printStackTrace
• Remove extra toString call, pass CharSequence to ClientMessagePacker

https://issues.apache.org/jira/browse/IGNITE-28310

[Copilot]

Pull request overview

This PR optimizes server-to-client error serialization by simplifying stack trace extraction and reducing allocations when packing stack traces into MsgPack messages (used by the thin client protocol).

Changes:

• Simplify ExceptionUtils.getFullStackTrace to rely on Throwable#printStackTrace and return a CharSequence to avoid an extra toString() allocation.
• Broaden ClientMessagePacker.packString to accept CharSequence, enabling direct packing of StringBuffer/StringBuilder without conversion.
• Update test fixture matcher to adapt to the new CharSequence return type.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
modules/core/src/testFixtures/java/org/apache/ignite/internal/testframework/matchers/CompletableFutureExceptionMatcher.java	Adjusts mismatch description building to handle CharSequence stack traces.
modules/core/src/main/java/org/apache/ignite/internal/util/ExceptionUtils.java	Reworks full stack trace extraction and changes the return type to CharSequence.
modules/client-common/src/main/java/org/apache/ignite/internal/client/proto/ClientMessagePacker.java	Updates packString API to accept CharSequence for lower-allocation packing.

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