Add OTEL_SEMCONV_EXCEPTION_SIGNAL_OPT_IN support to record_exception

[RKest]

Description

Span.record_exception currently always records exceptions as span events.
The semantic conventions for exceptions in logs and the span-event API deprecation plan (OTEP 4430, see also the Deprecating Span Events blog post) introduce the OTEL_SEMCONV_EXCEPTION_SIGNAL_OPT_IN environment variable so instrumentations can migrate exceptions from span events to logs.

This PR makes the SDK's Span.record_exception (and therefore the
__exit__ / use_span paths that call it) honor
OTEL_SEMCONV_EXCEPTION_SIGNAL_OPT_IN:

Value	Behavior
(unset)	Span events only — existing behavior (default).
logs	Record the exception as a log only.
logs/dup	Record the exception as both a log and a span event (phased rollout).

Any unrecognized value is treated as unset, preserving today's behavior.

Implementation notes:

• The exception log is emitted via the global LoggerProvider, correlated
  with the originating span's context (trace_id / span_id), at ERROR
  severity, with event_name="exception".
• The log carries exception.type, exception.message and
  exception.stacktrace. The deprecated exception.escaped attribute is
  intentionally omitted from the log representation; it is still set on the
  span event when one is recorded (logs/dup or default).
• The log is emitted under the span's own instrumentation scope.
• A new OTEL_SEMCONV_EXCEPTION_SIGNAL_OPT_IN constant is added to
  opentelemetry.sdk.environment_variables.

The default behavior is unchanged, so this is fully backwards compatible and
opt-in.

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

A new test module opentelemetry-sdk/tests/trace/test_record_exception_logs.py
covers all modes end to end using an in-memory span exporter and a captured
logger:

• unset / unrecognized values record a span event and emit no log
• logs records a log only (no span event)
• logs/dup records both a log and a span event
• the emitted log is correlated with the originating span
  (trace_id / span_id)
• the log uses the span's instrumentation scope
• exception.type / exception.message / exception.stacktrace are
  populated and extra attributes are forwarded
• the deprecated exception.escaped attribute is not set on the log

The existing record_exception tests in
opentelemetry-sdk/tests/trace/test_trace.py continue to pass unchanged.

uv run pytest opentelemetry-sdk/tests/trace/test_record_exception_logs.py
uv run pytest opentelemetry-sdk/tests/trace/test_trace.py -k record_exception

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No. The record_exception signature is unchanged; the new behavior is
  gated behind an environment variable and defaults to today's behavior.

Checklist:

• Followed the style guidelines of this project (ruff check / ruff format)
• Changelogs have been updated (.changelog/5318.added — rename to match
  the assigned PR number when opening the PR)
• Unit tests have been added
• Documentation has been updated (env var docstring + record_exception
  docstring)