Add durable_function_first_invocation tag to aws.lambda spans

[lym953]

Summary

• Adds a durable_execution() decorator in datadog_lambda/durable.py that wraps durable orchestration functions
• On each invocation, checks not context.state.is_replaying() and sets the durable_function_first_invocation tag ("true" or "false") on the active span
• Errors during tag setting are caught and logged at debug level to avoid disrupting user functions

Test plan

• test_sets_first_invocation_true_when_not_replaying — tag is "true" when context is not replaying
• test_sets_first_invocation_false_when_replaying — tag is "false" when context is replaying
• test_does_not_set_tag_when_no_active_span — no error when span is None
• test_does_not_raise_when_context_has_no_state — graceful handling of unexpected context shape
• test_preserves_function_name — functools.wraps is applied correctly
• Run pytest tests/test_durable.py — all 18 tests pass

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