Add stacktrace property to LogEntry (openEO API v1.3)

[Copilot]

openEO API v1.3 added a stacktrace field to log entries (e.g. from GET /jobs/{job_id}/logs) to provide structured stack trace information separate from the log message.

Changes

• LogEntry.stacktrace property — returns the stacktrace string from the entry, or None if absent
• Docstring — documents the new field alongside existing fields
• Tests — covers both the absent and present cases

log = LogEntry(
    id="log01",
    level="error",
    message="Something went wrong",
    stacktrace="Traceback (most recent call last):\n  ...",
)
log.stacktrace  # "Traceback (most recent call last):\n  ..."

# Field absent → None
LogEntry(id="log01", level="error", message="oops").stacktrace  # None