fix: preserve chained exception cause in logException

[alexstandiford]

Problem

CanLogException::logException() logs only the outer exception's message and assigns the exception object to context['exception'] — it never walks getPrevious().

This became a silent data-loss bug once infrastructure exceptions started carrying their real cause as a chained previous exception. The PDO datastore backend (phpnomad/mysql-integration) deliberately throws:

throw new DatastoreErrorException('Failed to execute query.', 500, $e); // real PDOException chained as $previous

The generic outer message is intentional (SQL/driver detail must not reach clients), with the actual cause carried on the chain for loggers. But logException discarded the chain, so every datastore failure logged:

[CRITICAL] - Failed to execute query.

…with the real error (column not found, duplicate key, constraint violation) gone. Downstream, consumers like Siren have ~360 logException($e) callsites that all went blind at once. The exception object in context['exception'] doesn't help listeners that json_encode context — an Exception serializes to {}.

Fix

Walk the $e->getPrevious() chain and fold each non-empty message into the logged line. Consumer usage doesn't change — bare logException($e) now surfaces the real cause automatically.

Folding the whole chain naively has two failure modes, both handled:

• Duplication — some wrappers embed the cause's message in their own text and chain the cause (e.g. QueryStrategy: 'Invalid query: ' . $e->getMessage() with $e as previous). A level is skipped when its message is already contained in a collected part, so it isn't logged twice.
• Unbounded growth — the walk is bounded by a depth cap ($maxLoggedExceptionDepth, default 10) plus a cycle guard, so a pathological chain can't produce a giant message.

Also drops the spurious leading " - " separator that appeared when no prefix message was supplied.

Before:

 - Failed to execute query.

After:

Failed to execute query. - SQLSTATE[42S22]: Column not found: 1054 Unknown column "foo"

Tests

Adds CanLogExceptionTest (11 tests): default/explicit level, context preservation, single- and multi-level chain flattening, the duplication-skip case, the depth cap, and the leading-separator fix. Full suite green; cs-fixer clean.

Version bumped `1.2.1 → 1.2.2`.