Logs Enabled: implement Development-status conditions (minimum_severity, trace_based) #5525

What

Implement the remaining, Development-status conditions of the Logs Enabled spec in the SDK: severity-based filtering (minimum_severity) and trace-based filtering (trace_based).

Background

PR #5380 adds Logger.enabled() and implements the stable conditions of the spec's Enabled section:

• there are no registered LogRecordProcessors
• LoggerConfig.enabled is false (handled in Logger.enabled())
• all registered LogRecordProcessors implement Enabled and each returns false

The spec lists two further conditions under which Enabled MUST return false, both currently marked [Development]:

• the provided severity is specified (not 0) and is less than the configured minimum_severity
• trace_based is true and the current context is associated with an unsampled trace

These were intentionally deferred out of #5380 (see #5380 (comment) and the issue discussion at #5360 (comment)).

Why deferred

• Both conditions are still Development status in the spec and subject to change.
• _LoggerConfig currently only has is_enabled; these conditions require adding minimum_severity and trace_based fields to the config model first. The tracer analog _TracerConfig also only has is_enabled, so the config model should grow consistently across signals (related: Public API for setting logger/tracer configuration programmatically #5524).

Proposed scope

• Add minimum_severity and trace_based fields to _LoggerConfig.
• Have Logger.enabled() return false when the provided severity is below minimum_severity, and when trace_based is set and the current context is an unsampled trace.
• Track the spec closely, since these conditions are Development status.

cc @aabmass @herin049