fix(python): preserve real tracing target in pylance log passthrough

[dpruijt]

Summary

The pylance tracing → log passthrough hard-coded the log record target to "lance::events" for every event, so the env_logger backend (filtered via LANCE_LOG) could only see one target. This made it impossible to filter individual event families by target — e.g. the high-volume per-file lance::file_audit delete events could only be killed by dropping the level globally, which also silences useful INFO logs like cleanup progress.

This change forwards each event under its real tracing target instead:

// before: every event collapses to one target
log::log!(target: "lance::events", state.level, "target=\"{}\" {}", event.metadata().target(), fields.str);

// after: preserve the originating target
let event_target = event.metadata().target();   // e.g. "lance::file_audit", "lance::dataset::cleanup"
log::log!(target: event_target, state.level, "target=\"{}\" {}", event_target, fields.str);

env_logger filters by the log record's target, so preserving it makes per-target LANCE_LOG directives work as expected.

Motivating use case (Databricks Lance cleanup jobs): suppress the one-line-per-file lance::file_audit spam while keeping the bulk lance::dataset::cleanup progress summaries.

Verified locally on a wheel built from this branch:

• LANCE_LOG="info" → file_audit lines present + progress present (baseline, both under their real targets now).
• LANCE_LOG="info,lance::file_audit=warn" → file_audit lines = 0, Delete phase complete / manifest-progress lines still present.

The text field target="..." in the formatted message is unchanged for backwards compatibility with any log scraping that keys off it.

Link to Devin session: https://app.devin.ai/sessions/b43ab1ec117947eb88c1e77aa17215b9
Requested by: @dpruijt

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